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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2019	Mar. 06, 2020	Jun. 28, 2019
Document and Entity Information
Document Type	10-K
Document Period End Date	Dec. 31, 2019
Entity Registrant Name	Akero Therapeutics, Inc.
Entity Well Known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		28,607,913
Entity Public Float			$ 195,939,947
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001744659
Amendment Flag	false

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 64,788	$ 75,975
Short-term marketable securities	71,612	0
Prepaid expenses and other current assets	1,649	1,156
Total current assets	138,049	77,131
Other assets	69	20
Total assets	138,118	77,151
Current liabilities:
Accounts payable	947	1,373
Accrued expenses and other current liabilities	8,422	969
Total current liabilities	9,369	2,342
Other liabilities	23
Total liabilities	9,392	2,342
Commitments and contingencies (Note 12)
Redeemable convertible preferred stock (Series A and B), $0.0001 par value; no shares authorized, issued and outstanding as of December 31, 2019; 64,730,410 shares authorized, issued and outstanding as of December 31, 2018; aggregate liquidation preference of $0 and $96,358 as of December 31, 2019 and December 31, 2018, respectively		124,728
Stockholders’ equity (deficit):
Common stock, $0.0001 par value, 150,000,000 shares authorized as of December 31, 2019 and 75,000,000 shares authorized as of December 31, 2018; 28,567,837 and 238,986 shares issued and outstanding as of December 31, 2019 and December 31, 2018, respectively	3
Additional paid-in capital	259,049	36,646
Accumulated other comprehensive loss	(6)
Accumulated deficit	(130,320)	(86,565)
Total stockholders’ equity (deficit)	128,726	(49,919)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 138,118	$ 77,151

Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Consolidated Balance Sheets
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	64,730,410
Redeemable convertible preferred stock, shares issued	0	64,730,410
Redeemable convertible preferred stock, shares outstanding	0	64,730,410
Aggregate liquidation preference stock, Value	$ 0	$ 96,358
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	75,000,000
Common stock, shares issued	28,567,837	238,986
Common stock, shares outstanding	28,567,837	238,986

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 37,046	$ 11,882
General and administrative	8,605	1,896
Total operating expenses	45,651	13,778
Loss from operations	(45,651)	(13,778)
Other income (expense), net:
Change in fair value of preferred stock tranche obligation		(62,150)
Change in fair value of anti-dilution right liability		(5,765)
Other income, net	1,896
Other expense, net		(21)
Total other income (expense), net	1,896	(67,936)
Net loss	(43,755)	(81,714)
Accretion of redeemable convertible preferred stock to redemption value		(520)
Net loss attributable to common stockholders	$ (43,755)	$ (82,234)
Net loss per share attributable to common stockholders - basic and diluted	$ (2.90)	$ (795.28)
Weighted-average number of shares used in computing net loss per share attributable to common stockholders, basic and diluted	15,070,728	103,403
Net unrealized loss on marketable securities	$ (6)
Comprehensive loss	$ (43,761)	$ (81,714)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Redeemable Convertible Preferred Stock	Series A Preferred Stock Second Tranche Closing	Series A Preferred Stock	Series B Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In-Capital	Other Comprehensive Loss	Accumulated Deficit	Total
Balance at beginning at Dec. 31, 2017	$ 5,000								$ (4,564)	$ (4,564)
Balance at beginning (in shares) at Dec. 31, 2017	5,000,000				226,400
Repurchase of founders’ stock (in shares)					(75,467)	75,467
Issuance of treasury stock as founders’ stock (in shares)					75,467	(75,467)
Issuance of stock		$ 24,996	$ 8,787	$ 45,271
Issuance of stock (in shares)		25,000,001	17,653,333	13,871,948
Settlement of future purchase obligation	$ 32,750									(32,750)
Issuance of Series A redeemable convertible preferred stock, in settlement of anti dilution right liability			$ 7,404
Issuance of Series A redeemable convertible preferred stock, in settlement of anti dilution right liability, ( in shares)			3,205,128
Extinguishment of call option liability							$ 36,750			36,750
Exercise of stock options							8			$ 8
Exercise of stock options (in shares)					12,586					12,586
Stock-based compensation expense							121			$ 121
Accretion of redeemable convertible preferred stock to redemption value	520						(233)		(287)	(520)
Net loss									(81,714)	(81,714)
Balance at ending at Dec. 31, 2018	$ 124,728						36,646		(86,565)	(49,919)
Balance at ending (in shares) at Dec. 31, 2018	64,730,410				238,986
Conversion of convertible preferred stock into common stock	$ 124,728				$ 2		(124,726)			(124,728)
Conversion of convertible preferred stock into common stock upon closing of public offering (in shares)	(64,730,410)				21,056,136
Issuance of stock					$ 1		95,452			95,453
Issuance of stock (in shares)					6,612,500
Issuance of restricted common stock upon early exercise of stock options (in shares)					491,207
Exercise of stock options							130			$ 130
Exercise of stock options (in shares)					164,503					655,710
Vesting of restricted common stock							240			$ 240
Issuance of common stock pursuant to ESPP purchases							85			85
Issuance of common stock pursuant to ESPP purchases (in shares)					4,505
Stock-based compensation expense							1,770			1,770
Net unrealized loss on marketable securities								$ (6)		(6)
Net loss									(43,755)	(43,755)
Balance at ending at Dec. 31, 2019					$ 3		$ 259,049	$ (6)	$ (130,320)	$ 128,726
Balance at ending (in shares) at Dec. 31, 2019					28,567,837

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Net issuance costs and underwriting fees	$ 10,348

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (43,755)	$ (81,714)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,770	121
Shares issued in connection with Amgen Agreement		1,353
Acquisition of technology in connection with Amgen Agreement		5,000
Issuance date fair-value of anti-dilution liability		1,639
Change in fair value of preferred stock tranche liability		62,150
Change in fair value of anti-dilution right liability		5,765
Net amortization of premiums and discounts on short-term investments	(104)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(507)	(1,059)
Accounts payable	(426)	1,313
Accrued expenses and other current liabilities	7,395	807
Net cash used in operating activities	(35,627)	(4,625)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of short-term marketable securities	(71,513)
Acquisition of technology in connection with Amgen Agreement		(5,000)
Net cash used in investing activities	(71,513)	(5,000)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock in initial public offering, net of issuance costs and underwriting fees	95,452
Proceeds from the early exercise of stock options in exchange for restricted common stock	321
Proceeds from the exercise of stock options	130	8
Proceeds from the issuance of common stock pursuant to employee stock purchase plan purchases	85
Payment of initial public offering costs		(52)
Payment of preferred stock issuance costs		(449)
Net cash provided by financing activities	95,988	85,007
Net (decrease) increase in cash, cash equivalents and restricted cash	(11,152)	75,382
Cash and restricted cash at the beginning of the year	76,000	618
Cash, cash equivalents and restricted cash at the end of the year	64,848	76,000
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Conversion of convertible preferred stock into common stock	124,728
Net unrealized loss on marketable securities	$ (6)
Accretion of redeemable convertible preferred stock to redemption value		520
Issuance date fair value of preferred stock tranche obligation		7,350
Deferred offering costs included in accounts payable and accrued expenses and other current liabilities		309
Shares issued in connection with Amgen Agreement		(1,353)
Settlement of future purchase obligation		32,750
Extinguishment of call option liability		36,750
Series A Preferred Stock
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of redeemable convertible preferred stock		40,000
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Issuance of Series A redeemable convertible preferred stock in settlement of anti dilution right liability		7,404
Series B Preferred Stock
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of redeemable convertible preferred stock		$ 45,500

Nature of the business and basis of presentation	12 Months Ended
Nature of the business and basis of presentation	Dec. 31, 2019
Nature of the business and basis of presentation
Nature of the business and basis of presentation	1. Nature of the business and basis of presentation Akero Therapeutics, Inc., together with its wholly owned subsidiary Akero Securities Corporation, (“Akero” or the “Company”) is a clinical-stage biotechnology company dedicated to developing pioneering medicines that restore metabolic balance and improve overall health for patients with nonalcoholic steatohepatitis, or NASH. NASH is a severe form of nonalcoholic fatty liver disease, or NAFLD, characterized by inflammation and fibrosis in the liver that can progress to cirrhosis, liver failure, cancer and death. Our lead product candidate is AKR‑001, an analog of fibroblast growth factor 21 (“FGF21”). We are currently conducting a Phase 2a clinical trial, the BALANCED study, which is evaluating AKR-001 in the treatment of NASH patients. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, ability to secure additional capital to fund operations, completion and success of clinical testing, compliance with governmental regulations, development by competitors of new technological innovations, dependence on key personnel and protection of proprietary technology. AKR‑001 will require extensive clinical testing prior to regulatory approval and commercialization. These efforts require significant amounts of additional capital, adequate personnel, and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Basis of presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company after elimination of all intercompany accounts and transactions. All adjustments necessary for the fair presentation of the Company’s consolidated financial statements for the periods have been reflected. Initial public offering On June 24, 2019, Akero completed its initial public offering or IPO at which time the Company issued 6,612,500 shares of common stock, including the exercise in full by the underwriters of their option to purchase up to 862,500 additional shares of common stock, at a public offering price of $16.00 per share. The Company received $98,394, net of underwriting discounts and commissions, but before deducting offering costs payable by the Company, which were $2,942. Upon the closing of the IPO, all outstanding shares of redeemable convertible preferred stock converted into 21,056,136 shares of common stock (see Note 6). In connection with the completion of its IPO in June 2019, the Company amended its certificate of incorporation to authorize the issuance of up to 150,000,000 shares of $0.0001 par value common stock and 10,000,000 shares of $0.0001 par value preferred stock designated as undesignated preferred stock. Reverse stock split On June 6, 2019, the Company effected a one-for-3.07418 reverse stock split of the Company’s common stock. All common stock, stock options and per share information presented have been adjusted to reflect the reverse stock split on a retroactive basis for all periods presented. There was no change in the par value of the Company’s common stock. The ratio by which shares of preferred stock are convertible into shares of common stock was adjusted to reflect the effects of the reverse stock split. Liquidity In accordance with Accounting Standards Update (“ASU”) No. 2014‑15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205‑40), the Company has evaluated whether there are certain conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. Since its inception, the Company has funded its operations primarily with proceeds from sales of redeemable convertible preferred stock and most recently with proceeds from the IPO. The Company has incurred recurring losses since its inception, including a net loss of $43,755 and $81,714 for the years ended December 31, 2019 and 2018, respectively. In addition, as of December 31, 2019, the Company had an accumulated deficit of $130,320. The Company expects to continue to generate operating losses for the foreseeable future. As of March 16, 2020, the issuance date of these consolidated financial statements, the Company expects that its existing cash, cash equivalents and short-term marketable securities of $136,400 as of December 31, 2019, will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of these consolidated financial statements. The Company expects that it will require additional funding beyond this time to complete the clinical development of AKR‑001, commercialize AKR‑001, if it receives regulatory approval, and pursue in-licenses or acquisitions of other product candidates. If the Company is unable to obtain funding, the Company will be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

Summary of significant accounting policies	12 Months Ended
Summary of significant accounting policies	Dec. 31, 2019
Summary of significant accounting policies
Summary of significant accounting policies	2. Summary of significant accounting policies Use of estimates The preparation of the Company's consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuations of common stock, preferred stock tranche obligation, anti-dilution right liability and the valuation allowance for deferred tax assets. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. Cash and cash equivalents The Company considers all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value. Short-term marketable securities The Company invests in short-term marketable securities, primarily money market funds, commercial paper, U.S. treasury securities and corporate debt securities. The Company classifies its short-term marketable securities as available-for-sale securities and reports them at fair value in short-term marketable securities on the consolidated balance sheets with related unrealized losses included within accumulated other comprehensive loss on the consolidated balance sheets. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in other income (expense), net on the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income (expense), net. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in other income (expense), net. The Company regularly reviews all its investments for other-than-temporary declines in estimated fair value. This review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. When the Company determines that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, the carrying value of the security will be reduced and a loss will be recorded for the amount of such decline. Restricted cash As of December 31, 2019 and 2018, the Company was required to maintain separate cash balances of $40 and $20, respectively, to collateralize corporate credit cards with a bank, which are classified within other assets (non-current) on the consolidated balance sheets. As of December 31, 2019 the Company was required to maintain a separate cash balance of $20 for the benefit of the landlord in connection with the Company’s office space lease in South San Francisco, California (the “Lease”), which is classified within other assets (non-current) on the 2019 consolidated balance sheet (see Note 12). As of December 31, 2018, the Company was required to maintain a separate cash balance of $5 for the benefit of the landlord in connection with the Lease, which is classified within other current assets on the 2018 consolidated balance sheet. Concentrations of credit risk Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents and short-term marketable securities. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company deposits its cash investments in financial institutions that it believes have high credit quality and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships. At December 31, 2019 and 2018, all of the Company's cash, cash equivalents and short-term investments were held at one accredited financial institution. Deferred offering costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process preferred stock or common equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded as a reduction to the carrying value of redeemable convertible preferred stock or in stockholders' equity (deficit) as a reduction of additional paid-in capital generated as a result of such offering. Should an in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive loss. As of December 31, 2019, the Company did not have any deferred offering costs. As of December 31, 2018, the Company recorded deferred offering costs of $361, which are classified within prepaid expenses and other current assets on the 2018 consolidated balance sheet. Segment information The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company's singular focus is developing and commercializing transformative treatments for serious metabolic diseases, with an initial focus on NASH. Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred to discover, research and develop drug candidates, including personnel expenses, stock-based compensation expense, third-party license fees and external costs including fees paid to consultants and clinical research organizations ("CROs"), in connection with drug product manufacturing, nonclinical studies and clinical trials, and other related clinical trial fees, such as for investigator grants, patient screening, laboratory work, clinical trial database management, clinical trial material management and statistical compilation and analysis. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. Costs incurred in obtaining technology licenses are charged immediately to research and development expense if the technology licensed has not reached technological feasibility and has no alternative future uses. Research contract costs and accruals The Company has entered into various research and development and other agreements with commercial firms, researchers and others for provisions of goods and services. These agreements are generally cancelable, and the related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ materially from the Company's estimates. Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock-based compensation The Company measures all stock-based awards granted to employees and nonemployees based on the fair value on the date of the grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award, on a straight-line basis. The Company accounts for forfeitures as they occur. The Company estimates the fair value of stock option grants using the Black-Scholes option pricing model. Prior to our initial public offering, the exercise price for all stock options granted was at the estimated fair value of the underlying common stock as determined on the date of grant by the Company’s board of directors. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model, which requires inputs based on certain subjective assumptions, including the expected stock price volatility, the expected term of the option, the risk-free interest rate for a period that approximates the expected term of the option, and the Company's expected dividend yield. The Company went public in June 2019 and accordingly, lacks sufficient company-specific historical and implied volatility information for its shares traded in the public markets. Therefore, it estimates its expected share price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The expected term of the Company's stock options has been determined utilizing the "simplified" method for awards that qualify as "plain-vanilla" options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. The fair value of each restricted common stock award is estimated on the date of grant based on the fair value of the Company's common stock on that same date. Compensation expense for purchases under the Employee Stock Purchase Plan is recognized based on the fair value of the common stock estimated based on the closing price of our common stock as reported on the date of offering, less the purchase discount percentage provided for in the plan. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified. Preferred stock tranche obligation The Company classified the preferred stock tranche obligation for the future purchase, and option to purchase, Series A Preferred Stock (see Note 6) as a liability on its consolidated balance sheets as the preferred stock tranche obligation was a freestanding financial instrument that required the Company to transfer equity instruments upon future closings of the Series A Preferred Stock. The preferred stock tranche obligation was initially recorded at fair value upon the date of issuance and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred stock tranche obligation were recognized as a component of other expense in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the preferred stock tranche obligation were recognized until the tranche obligations were fulfilled or otherwise extinguished in the fourth quarter of 2018. In November 2018, in connection with the Company's issuance and sale of Series A Preferred Stock, the Company satisfied its obligation to issue additional shares under the Second Tranche Closing. In December 2018, in connection with the Company's issuance and sale of Series B Preferred Stock, the Company terminated the option to purchase Series A Preferred Stock provided under the 2018 Series A Agreement (see Notes 3 and 6). Anti-dilution right liability The Company classified the anti-dilution right under its license agreement with Amgen Inc. ("Amgen") (see Note 9) as a derivative liability on its consolidated balance sheets as the anti-dilution right represented a freestanding financial instrument that required the Company to transfer equity instruments upon future equity closings. The anti-dilution right liability was initially recorded at fair value upon the date of issuance and was subsequently remeasured to fair value at each reporting date. The issuance date fair value of the anti-dilution right liability was recognized as a research and development expense upon entering into the agreement with Amgen. Changes in the fair value of the anti-dilution right liability were recognized as a component of other expense in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the anti-dilution right liability were recognized until the anti-dilution right was satisfied in the fourth quarter of 2018. In November 2018, in connection with the Company’s issuance and sale of Series A Preferred Stock, the Company satisfied its anti-dilution right under the Amgen Agreement (see Notes 6 and 9). Classification and accretion of redeemable convertible preferred stock The Company has classified its redeemable convertible preferred stock outside of stockholders' equity (deficit) because the shares contain certain redemption features that are not solely within the control of the Company. Costs incurred in connection with the issuance of redeemable convertible preferred stock, as well as the recognition of the preferred stock tranche obligation, are recorded as a reduction of gross proceeds from issuance. The net carrying value of redeemable convertible preferred stock were accreted to their redemption values through a charge to additional paid-in capital or accumulated deficit over the period from date of issuance to the earliest date on which the holders could, at their option, elect to redeem their shares. In December 2018, in connection with the Company's issuance and sale of Series B Preferred Stock, the Company terminated the redemption rights associated with the Series A Preferred Stock that allowed the holders, at their option, to elect to redeem their shares at a specified date. Accordingly, the Company ceased accreting the net carrying value of the Series A redeemable convertible preferred stock to the redemption value. Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders' equity (deficit) that result from transactions and economic events other than those with stockholders. Our comprehensive loss is comprised of net loss and changes in unrealized gains and losses on our short-term marketable securities. Income taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company's tax returns. Deferred tax assets and liabilities are determined based on the difference between the consolidated financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Net loss per share The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. For purpose of this calculation, unvested restricted common stock and redeemable convertible preferred stock are considered potential dilutive common shares. The Company’s redeemable convertible preferred stock contractually entitles the holders of such shares to participate in dividends but does not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2019 and 2018. Emerging growth company The Company is an emerging growth company as defined in the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”). Under the JOBS Act, companies have extended transition periods available for complying with new or revised accounting standards. The Company has elected this exemption to delay adopting new or revised accounting standards until such time as those standards apply. Recently issued accounting pronouncements not yet adopted In February 2016, the FASB issued ASU No. 2016‑02, Leases (Topic 842), subsequently amended by ASU 2018-10, ASU 2018-11, ASU 2019-01 and ASU 2019-10 (collectively, “ASU 2016‑02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification on the consolidated balance sheets. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. The Company intends to utilize the modified retrospective approach to adopting ASU 2016‑02 effective January 1, 2020. Further, the Company intends to utilize the package of available practical expedients which allows it to i) not reassess whether any expired or existing contracts are or contain leases; ii) not reassess the lease classification for expired or existing leases; and iii) not reassess the treatment of initial direct costs for any existing leases. The Company is in the process of completing a review of its existing lease agreements under ASC 842 and does not expect that the impact of the adoption of ASU 2016-02 on its consolidated balance sheets will be material and does not expect the adoption to have a material impact on its results of operations or cash flows. In February 2020 the Company entered into a new office lease agreement (see Note 15). The Company is in the process of determining the impact on its consolidated financial statements of the adoption of ASU 2016-02 related to this new lease. In August 2018, the FASB issued No. ASU 2018‑13, Fair Value Measurement (Topic 820)—Disclosure Framework (“ASU 2018‑13”), which improves the disclosure requirements for fair value measurements. For non-public entities, ASU 2018‑13 is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted for any removed or modified disclosures. The Company is currently evaluating the impact that the adoption of ASU 2018‑13 will have on its consolidated financial statements.

Fair value of financial assets and liabilities

12 Months Ended

Dec. 31, 2019

Fair value of financial assets and liabilities

Fair value of financial assets and liabilities

3. Fair value of financial assets and liabilities

Certain assets and liabilities of the Company are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

Level 1—Quoted prices in active markets for identical assets or liabilities.

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The following table summarizes our financial assets measured at fair value on a recurring basis as of December 31, 2019:


	Fair Value Measurements as of December 31, 2019 Using:
	Total		Level 1		Level 2		Level 3
Money market funds	$	49,948	$	49,948	$	—	$	—
Commercial paper		49,114		—		49,114		—
U.S. treasury securities		6,048		6,048		—		—
Corporate debt securities		20,143		—		20,143		—
	$	125,253	$	55,996	$	69,257	$	—

At December 31, 2018, the Company did not have any financial assets or liabilities measured at fair value on a recurring basis. The carrying values of the Company’s prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.

In November 2018, in connection with the Company’s issuance and sale of Series A Preferred Stock under the Second Tranche Closing, the Company satisfied its obligation to issue additional shares at the Second Tranche Closing and accordingly reclassified the carrying value of the preferred stock tranche obligation associated with the future purchase obligation, equal to the then current fair value of $32,750, to the carrying value of the Series A Preferred Stock. In November 2018, in connection with the Company’s Second Tranche Closing, the Company issued 3,205,128 shares of Series A Preferred Stock to Amgen for a total value of $7,404 satisfying its anti‑dilution obligation under the Amgen Agreement. The Company reclassified the carrying value of the anti‑dilution right liability, equal to the then current fair value of $7,404, to the carrying value of the Series A Preferred Stock.

In December 2018, in connection with the Company’s issuance and sale of Series B Preferred Stock, the Company terminated the option to purchase Series A Preferred Stock provided under the 2018 Series A Agreement. The Company accounted for the termination of the call option associated with the preferred stock tranche obligation as a liability extinguishment between related parties and recognized a gain on extinguishment of $36,750, equal to the then current fair value, within additional paid‑in capital in the statement of stockholder’s equity (deficit).

During the years ended December 31, 2019 and December 31, 2018, there were no transfers between Level 1, Level 2 and Level 3.

Valuation of cash equivalents and short-term investments

Commercial paper and corporate debt securities were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy.

Valuation of preferred stock tranche obligation

The fair value of the preferred stock tranche obligation recognized in connection with the Company’s issuance of Series A Preferred Stock in June 2018 (see Note 6) was determined based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy. The fair value of the liability was estimated based on results of a third‑party valuation performed in connection with the June 2018 Series A issuance. The Company determined that this valuation represented the fair value of the liability at the reporting date. The liability included (i) an obligation to issue shares in a second tranche of Series A Preferred Stock and (ii) an obligation to issue shares under the call option to purchase Series A Preferred Stock following the Second Tranche Closing.

The fair value of the obligation to purchase a second tranche of Series A Preferred Stock was estimated by utilizing the future value of the underlying Series A Preferred stock, the Series A original issue price and the number of shares subject to future purchase. The future value of the Series A Preferred Stock was determined through a backsolve calculation. The present value of the forward contract was then multiplied by a probability of occurrence for the Second Tranche Closing.

The fair value of the obligation for the call option to purchase Series A Preferred Stock was estimated using the hybrid model, which employed the Black‑Scholes option‑pricing model adjusted to reflect the timing and probability of closing a second tranche of Series A Preferred Stock. The hybrid method incorporates assumptions and estimates, to value the obligation. Estimates and assumptions impacting the fair value measurement include the fair value of the underlying shares of Series A Preferred Stock, the remaining contractual term of the preferred stock tranche obligation, risk‑free interest rate, expected dividend yield, expected volatility of the price of the underlying preferred stock, the remaining years to liquidity, the discount rate and probability (expressed as a percentage) of closing a second tranche. The most significant assumption in the hybrid model impacting the fair value of the call option is the fair value of the preferred stock as of each remeasurement date. The Company determined the fair value per share of the underlying preferred stock by taking into consideration the most recent sales of preferred stock, results obtained from third‑party valuations and additional factors that are deemed relevant. The Company has historically been a private company and lack company‑specific historical and implied volatility information of its stock. Therefore, the Company estimated expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the call option. The risk‑free interest rate is determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining years to liquidity. The Company has estimated a 0% dividend yield based on the expected dividend yield and the fact that the Company has never paid or declared dividends.

Valuation of anti‑dilution right liability

The fair value of the anti‑dilution right liability recognized in connection with the anti‑dilution provisions set forth in the Company’s license agreement with Amgen (see Note 9) was determined based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy.

The fair value of the anti‑dilution right was estimated using a probability weighted scenario which considered as inputs the probability of occurrence of events that would trigger the issuance of shares, including a (i) Second Tranche Closing of Series A Preferred Stock, (ii) initial public offering, and (ii) no future sale of equity securities. The weighted average fair values of each scenario were calculated utilizing the fair value per share of the underlying Series A Preferred Stock and common stock. Changes in the estimated fair value and the probability of achieving different financing scenarios can have a significant impact on the fair value of the anti‑dilution right liability.

The following table presents a roll forward of the fair values of the Company’s preferred stock tranche obligation and anti‑dilution right liability for the year ended December 31, 2018, for which fair value is determined using Level 3 inputs:


	Preferred stock		Anti‑dilution
	tranche		right
	obligation		liability
Balance as of December 31, 2017	$	—	$	—
Initial fair value of anti‑dilution right liability in connection with Amgen license agreement		—		1,639
Initial fair value of preferred stock tranche obligation in connection with the issuance of Series A Preferred Stock		7,350		—
Change in fair value		62,150		5,765
Settlement of future purchase obligation		(32,750)		—
Settlement of anti‑dilution right liability upon issuance of Series A preferred stock		—		(7,404)
Extinguishment of call option liability		(36,750)		—
Balance as of December 31, 2018	$	—	$	—

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Short-term marketable securities

12 Months Ended

Dec. 31, 2019

Short-term marketable securities

Short-term marketable securities

4. Short-term marketable securities

The following is a summary of short-term marketable securities presented on the Company’s consolidated balance sheet as of December 31, 2019:


	December 31, 2019
			Gross		Gross
	Amortized		unrealized		unrealized		Fair
	Cost		gains		losses		value
Money market funds	$	49,948	$	—	$	—	$	49,948
Commercial paper		49,114		—		—		49,114
U.S. treasury securities		6,048		—		—		6,048
Corporate debt securities		20,149		—		(6)		20,143
	$	125,259	$	—	$	(6)	$	125,253

Cash and cash equivalents							$	53,641
Short-term marketable securities								71,612
							$	125,253

Accrued expenses and other current liabilities

12 Months Ended

Dec. 31, 2019

Accrued expenses and other current liabilities

Accrued expenses and other current liabilities

5. Accrued expenses and other current liabilities

Accrued expenses and other current liabilities consisted of the following:


	December 31,
	2019		2018
Accrued employee compensation and benefits	$	1,606	$	304
Accrued external research and development expenses		6,361		430
Accrued legal and professional fees		370		106
Other		85		129
	$	8,422	$	969

Redeemable convertible preferred stock

12 Months Ended

Dec. 31, 2019

Redeemable convertible preferred stock

Redeemable convertible preferred stock

6. Redeemable convertible preferred stock

As of December 31, 2019 and 2018, 0 shares and 64,730,410 shares, respectively, of redeemable convertible preferred stock were issued and outstanding and were classified outside of stockholders' equity (deficit) because the shares contained redemption features that were not solely within the control of the Company.

Upon completion of the Company’s IPO on June 24, 2019, all outstanding shares of our Series A and Series B redeemable convertible preferred stock (the “Preferred Stock”) were converted into 21,056,136 shares of common stock and the related carrying value was reclassified to common stock and additional paid-in capital. Accordingly, there were no shares of redeemable convertible preferred stock outstanding as of December 31, 2019.

In June 2018, the Company entered into a Series A Preferred Stock Agreement ("2018 Series A Agreement"), which provided for a First Tranche Closing, Second Tranche Closing and a call option to purchase additional shares of Series A Preferred Stock.

In June 2018, the Company completed its First Tranche Closing, and issued and sold 15,000,000 shares of Series A Preferred Stock at a price of $1.00 ("2018 Series A Agreement Purchase Price") per share for aggregate proceeds of $14,784, net of issuance costs of $216. Upon the execution of the Company's license agreement with Amgen (see Note 9), the Company issued an additional 2,653,333 shares of Series A Preferred Stock to Amgen in June 2018 for a total value of $1,353.

The Second Tranche Closing was contingent upon the achievement of the Second Tranche Closing Milestone Event ("Milestone Event"), as reasonably determined by a majority of the Company's board of directors, prior to December 31, 2019 or waiver of the Milestone Event through written elections by a majority of the purchasers of shares of Series A Preferred Stock. Upon achievement or waiver of the Milestone Event, Amgen would be issued a number of additional shares for no consideration provided that Amgen's total holdings equal ten percent (10%) of the total outstanding and issued common stock of the Company on a fully diluted and as converted basis of the Second Tranche Closing. Additionally, the 2018 Series A Agreement contained a call option such that, following the Second Tranche Closing, the stockholders of Series A Preferred Stock have the right, but not the obligation, to purchase up to $20,000 of additional shares of Series A Preferred Stock at any time prior to January 14, 2022 at a price equal to the 2018 Series A Agreement Purchase Price.

The Company concluded that the rights to participate in the future issuance of Series A Preferred Stock met the definition of a freestanding financial instrument that was required to be recognized as a liability at fair value as (i) the instruments were legally detachable from the Series A Preferred Stock and (ii) required the Company to transfer equity instruments upon future closings of the Series A Preferred Stock. Upon the First Tranche Closing, the Company recognized a preferred stock tranche obligation of $7,350 with a corresponding reduction to the carrying value of the Series A Preferred Stock.

Upon the First Tranche Closing in June 2018, the initial carrying value of the Series A Preferred Stock was recorded at $8,787, equal to $15,000 of gross proceeds received by the Company and the fair value of $1,353 for the issuance of shares to Amgen, reduced by accrued issuance costs of $216 and the fair value of the preferred stock tranche obligation of $7,350.

Upon issuance of each tranche of Preferred Stock, the Company assessed the embedded conversion and liquidation features of the securities and determined that such features did not require the Company to separately account for these features. The Company also concluded that no beneficial conversion feature existed on the issuance date of each tranche of Preferred Stock.

In November 2018, upon the waiver of the Milestone Event through written elections by a majority of the shareholders of Series A Preferred Stock, the Company closed the second tranche of its Series A preferred financing through the issuance and sale of an aggregate of 25,000,001 shares of Series A Preferred Stock, at an issuance price of $1.00 per share, for proceeds of $25,000, before issuance costs of $4. In November 2018, in connection with the Company's issuance and sale of Series A Preferred Stock, the Company satisfied its obligation to issue additional shares under the Second Tranche Closing. Accordingly, the preferred stock tranche obligation associated with the future purchase obligation was adjusted to fair value immediately prior to the issuance of the Series A Preferred Stock, and upon issuance of the Series A Preferred Stock, the preferred stock tranche obligation associated with the future purchase obligation of $32,750, equal to then current fair value, was reclassified to the carrying value of the Series A Preferred Stock.

In November 2018, pursuant to the terms of the Amgen Agreement, the Company issued an additional 3,205,128 shares of Series A Preferred Stock valued at $7,404 to Amgen upon completion of the Series A Second Tranche Closing, satisfying its anti-dilution right under the Amgen Agreement.

In December 2018, the Company issued and sold 13,871,948 shares of Series B Preferred Stock, at an issuance price of $3.28 per share, for proceeds of $45,500 before issuance costs of $229. In connection with the Company's issuance and sale of Series B Preferred Stock, the Company terminated the option to purchase Series A Preferred Stock provided under the 2018 Series A Agreement.

As of December 31, 2018, redeemable convertible preferred stock consisted of the following:


		Preferred
	Preferred	stock					Common stock
	stock	issued and	Carrying		Liquidation		issuable upon
	authorized	outstanding	value		preference		conversion
Series A Preferred Stock	50,858,462	50,858,462	$	79,457	$	50,858	16,543,739
Series B Preferred Stock	13,871,948	13,871,948	$	45,271	$	45,500	4,512,397
	64,730,410	64,730,410	$	124,728	$	96,358	21,056,136

Stockholder’s equity (deficit)

12 Months Ended

Dec. 31, 2019

Stockholder’s equity (deficit)

Stockholder’s equity (deficit)

7. Stockholders’ equity (deficit)

Common stock

As of December 31, 2019 and 2018, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 150,000,000 shares and 75,000,000 shares of $0.0001 par value common stock, respectively. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. The holders of common stock, voting exclusively and as a separate class, have the exclusive right to vote for the election of directors of the Company. Common stockholders are entitled to receive dividends, as may be declared by the board of directors. Through December 31, 2019, no cash dividends had been declared or paid.

On June 24, 2019, the Company completed its IPO at which time the Company issued 6,612,500 shares of common stock, including the exercise in full by the underwriters of their option to purchase up to 862,500 additional shares of common stock, at a public offering price of $16.00 per share. The Company received $98,394, net of underwriting discounts and commissions, but before deducting offering costs payable by the Company, which were $2,942. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 21,056,136 shares of common stock (see Note 6). As of December 31, 2019 and December 31, 2018, there were 28,567,837 and 238,986 shares of common stock issued and outstanding, respectively.

The following shares of common stock were reserved for issuance as follows:


	December 31,
	2019	2018
Conversion of outstanding shares of preferred stock	—	21,056,136
Options outstanding under the 2018 Stock Option and Grant Plan	2,296,029	1,839,913
Options outstanding under the 2019 Stock Option and Incentive Plan	800,526	—
Options available for future grant	1,771,931	1,219,461
2019 Employee Stock Purchase Plan	269,364	—
	5,137,850	24,115,510

Undesignated preferred stock

As of December 31, 2019, the Company’s fourth amended and restated certificate of incorporation authorized the Company to issue up to 10,000,000 shares of undesignated preferred stock, par value $0.0001 per share. There were no undesignated preferred shares issued or outstanding as of December 31, 2019.

Restricted common stock

In March 2017, the Company issued an aggregate of 226,400 shares of restricted common stock under restricted stock agreements with the founders. Pursuant to the terms of the agreements, the restricted common stock was initially subject to a vesting schedule over a four-year period commencing in January 2017 and culminating in January 2021. During the vesting period, the Company has the right to repurchase up to all unvested shares at the amount paid if the relationship between the recipient and the Company ceases. Subject to the continued employment or other business relationship with the Company, all of the restricted common stock becomes fully vested within four years of the date of issuance.

In October 2017, 75,467 shares of restricted common stock were subject to repurchase by the Company when one of the founders terminated his relationship with the Company. The Company repurchased the shares in March 2018 for an immaterial amount and immediately reissued the shares to the remaining founders. In connection with the repurchase and reissuance of the shares, the Company amended the restricted stock agreements with the remaining founders such that the restricted common stock is now subject to a vesting schedule over a two-year period commencing in May 2018 and culminating in May 2020.

The Company accounted for the acceleration of vesting under the amended restricted stock agreement as a modification of the original awards and recognized the remaining unvested shares prospectively over the revised vesting period. The grant date fair value of restricted stock vested during the years ended December 31, 2019 and 2018 was insignificant.

In April, June and July 2019, the Company amended certain option grant agreements granted under the Company’s 2018 Stock Option and Grant Plan to allow the holders the right to early exercise unvested options, subject to a repurchase right held by the Company equal to the lesser of the original exercise price per share or the fair value of the shares on the repurchase date. The unvested shares issued as a result of the early exercise are deemed restricted stock pursuant to a restricted stock agreement and a vesting schedule identical to the vesting schedule of the original grant agreement. The proceeds related to unvested restricted common stock are recorded as liabilities until the stock vests, at which point they are reclassified to additional paid-in capital. Common shares issued for the early exercise of options are included in issued and outstanding shares.

The following table summarizes restricted stock activity since December 31, 2017:


		Grant-Date Fair
	Number of Shares	Value
Unvested restricted common stock as of December 31, 2017	226,400	$	—
Shares vesting	(146,210)		—
Unvested restricted common stock as of December 31, 2018	80,190		—
Early exercise of unvested stock options	491,207		0.65
Shares vesting	(416,248)		0.65
Unvested restricted common stock as of December 31, 2019	155,149	$	0.65

As of December 31, 2019, there were 155,149 shares of unvested restricted common stock subject to repurchase, consisting of 23,598 shares from unvested restricted common stock awards under restricted stock agreements with the founders and 131,551 shares from the early exercise of stock options.

Stock-based awards

12 Months Ended

Dec. 31, 2019

Stock-based awards

Stock-based awards

8. Stock-based awards

2018 Stock option and grant plan

The Company’s 2018 Stock Option and Grant Plan (the “2018 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock awards and other stock-based awards to employees, directors and consultants of the Company. The 2018 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

The total number of shares of common stock that could have been issued under the 2018 Plan was 3,071,960 shares, of which 107,635 shares remained available for grant on June 18, 2019, the date that the Company’s 2019 Stock Option and Incentive Plan (the “2019 Plan”) became effective. Upon the effectiveness of the 2019 Plan, the 107,635 remaining shares available under the 2018 Plan were transferred and became available for issuance under the 2019 Plan. Shares of common stock underlying outstanding awards under the 2018 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) will be added to the shares of common stock available for issuance under the 2019 Plan.

2019 Stock option and incentive plan

The 2019 Plan was adopted and approved by the Company’s board of directors in May 2019 and by the Company’s stockholders in June 2019. The 2019 Plan became effective on June 18, 2019 and replaced the Company’s 2018 Plan on that date. The 2019 Plan allows the board of directors or the compensation committee of the board of directors to make equity-based incentive awards to the Company’s officers, employees, directors or other key persons (including consultants). The number of shares initially reserved for issuance under the 2019 Plan is 2,572,457, which includes the 107,635 shares transferred from the 2018 Plan, and shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors.

The 2019 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. All incentive options granted to any person possessing more than 10% of the total combined voting power of all classes of shares may not have an exercise price of less than 110% of the fair market value of the common stock on the grant date. Stock options granted to employees, officers, members of the board of directors and consultants will typically vest over a four-year period.

Shares that are expired, terminated, surrendered or canceled under the 2019 Plan without having been fully exercised will be available for future awards.

2019 Employee stock purchase plan

The 2019 Employee Stock Purchase Plan (the “2019 ESPP”) was adopted and approved by the Company’s board of directors in May 2019 and by the Company’s stockholders in June 2019. The 2019 ESPP became effective on June 18, 2019, at which time 273,869 shares were reserved for issuance. The 2019 ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2020 and each January 1 thereafter through January 1, 2029, by the least of (i) 1% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31, (ii) 410,803 shares or (iii) such number of shares as determined by the compensation committee.

Stock option valuation

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees, directors and consultants were as follows, presented on a weighted average basis:


	Year Ended December 31,
	2019			2018
Weighted average risk‑free interest rate		2.12	%		2.99	%
Expected term (in years)		6.0			6.0
Expected volatility		73.75	%		70.88	%
Expected dividend yield		0	%		0	%

Stock options

The following table summarizes the Company’s stock option activity since December 31, 2017:


		Weighted-		Weighted-
		Average		Average	Aggregate
		Exercise		remaining	Intrinsic
	Number	Price per		contractual	Value
	of Options	Share		term (years)	(000's)
Balance Outstanding, December 31, 2017	—	$	—	—	$	—
Options granted	1,883,067		0.61	9.74
Options exercised	(12,586)		0.61
Options cancelled	(30,568)		0.61
Balance Outstanding, December 31, 2018	1,839,913		0.61	9.74		10,577
Options granted	1,912,352		12.42
Options exercised	(655,710)		0.69
Balance Outstanding, December 31, 2019	3,096,555	$	7.89	9.19	$	44,323
Exercisable, December 31, 2019	175,300	$	3.54	8.88	$	3,270
Vested and expected to vest, December 31, 2019	3,096,555	$	7.89	9.19	$	44,323

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

The weighted average grant-date fair value per share of stock options granted during the year ended December 31, 2019 was $8.34

Stock-based compensation

The Company recorded $1,770 in stock-based compensation expense for the year ended December 31, 2019, with $485 classified as research and development expense and $1,285 classified as general and administrative expense in the consolidated statements of operations and comprehensive loss. The Company recorded $121 in stock-based compensation expense for the year ended December 31, 2018 , with $41 classified as research and development expense and $80 classified as general and administrative expense in the consolidated statements of operations and comprehensive loss.

As of December 31, 2019, total unrecognized compensation cost related to the unvested stock-based awards was $15,584, which is expected to be recognized over a weighted average period of 3.08 years.

In April, June and July 2019, certain option holders early exercised options to purchase 491,207 shares of common stock, at an average exercise price of $0.65 per share, for cash proceeds of $321 (See Note 7). Stock-based compensation expense related to these options will continue to be recognized over the requisite service period of the awards based on the grant-date fair value which was determined using the Black-Scholes option-pricing model.

Amgen license agreement	12 Months Ended
Amgen license agreement	Dec. 31, 2019
Amgen license agreement
Amgen license agreement	9. Amgen license agreement In June 2018, the Company entered into a license agreement (the “Amgen Agreement”) with Amgen pursuant to which the Company was granted an exclusive license to certain patents and intellectual property related to a long-acting FGF21 analog in order to commercially develop, manufacture, use and distribute FGF21 as a treatment for NASH and other serious metabolic diseases. The Amgen Agreement provides the Company with exclusive global rights to the licensed products and the right to grant sublicenses that cover AKR‑001 to third parties. In exchange for these rights, the Company made in 2018 an upfront payment of $5,000 and issued 2,653,333 shares of Series A Preferred Stock with a fair value of $1,353 to Amgen. The total consideration transferred to Amgen under the agreement of $6,353 is included within research and development expense in the consolidated statements of operations and comprehensive loss for 2018. The Company accounted for the acquisition of technology as an asset acquisition because it did not meet the definition of a business. The Company recorded the total consideration transferred to Amgen as research and development expense in the consolidated statements of operations and comprehensive loss for 2018 because the acquired technology represented in-process research and development and had no alternative future use. In addition, under the Amgen Agreement, Amgen was entitled to maintain a 10% ownership interest of the outstanding shares of the Company’s common stock, on a fully diluted and converted basis, through the second closing of the Company’s Series A Preferred Stock financing. The Company assessed the Amgen anti-dilution right and determined that the right (i) met the definition of a freestanding financial instrument that was not indexed to the Company’s own stock and (ii) met the definition of a derivative and did not qualify for equity classification. The anti-dilution right liability was initially valued at $1,639 which the Company recorded as research and development expense in June 2018. Changes in the fair value of the anti-dilution right liability continued to be recognized until the Company satisfied the obligation which occurred in November 2018. The Company recognized expense of $5,765 within other expense in the consolidated statements of operations and comprehensive loss for the year ended December 31, 2018, related to the change in fair value of the anti-dilution right liability prior to its extinguishment in November 2018. In November 2018, in connection with the second closing of the Company’s Series A Preferred Stock financing, the Company issued 3,205,128 shares of Series A Preferred Stock to Amgen for a total value of $7,404 satisfying its anti-dilution obligation under the Amgen Agreement. At the time, the Company reclassified the carrying value of the anti-dilution right liability, equal to the then current fair value of $7,404, to the carrying value of the Series A Preferred Stock. Under the Amgen Agreement, the Company made a milestone payment of $2,500 in connection with dosing the first patient in our Phase 2a clinical trial and is obligated to make aggregate remaining milestone payments to Amgen of up to $37,500 upon the achievement of specified clinical and regulatory milestones and aggregate milestone payments of up to $75,000 upon the achievement of specified commercial milestones for all products licensed under the agreement. Under the Amgen Agreement, the Company is obligated to pay Amgen tiered royalties ranging from a low to high single-digit percentages on annual net sales of the licensed products, beginning on the first commercial sale of such licensed products in each country and expiring on a country-by-country basis on the latest of (i) the expiration of the last valid patent claim covering such licensed products in such country, (ii) the loss of regulatory exclusivity in such country, and (iii) ten years after the first commercial sale of such licensed product in such country. The royalty payments are subject to reduction under specified conditions set forth in the agreement. The Company is solely responsible for all development, manufacturing, and commercial activities and costs of the licensed products, including clinical studies or other tests necessary to support the use of a licensed product. The Company is also responsible for costs related to the filing, prosecution and maintenance of the licensed patent rights. The Amgen Agreement will remain in effect until the expiration of the royalty term in all countries for all licensed products. The agreement may be terminated by either party with at least 90 days' notice in the event of material breach by the other party that remains uncured for 90 days, by either party for insolvency or bankruptcy of the other party and immediately by Amgen if the Company challenges the licensed patents. The Company may also terminate the agreement with 90 days' written notice for discretionary reasons such as scientific, technical, regulatory or commercial issues, as defined in the agreement. During the year ended December 31, 2019, the Company recorded research and development expense of $2,500 related to the achievement of a clinical milestone, as specified in the agreement. During the year ended December 31, 2018, the Company recorded research and development expense of $8,016 in connection with the Amgen Agreement, including the upfront cash payment of $5,000, the fair value of $1,353 of shares of Series A Preferred Stock issued to Amgen, the fair value of $1,639 for the issuance of the anti-dilution right liability and $24 of other research and development expenses.

Income taxes

12 Months Ended

Dec. 31, 2019

Income taxes

Income taxes

10. Income taxes

During the years ended December 31, 2019 and 2018, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each period due to its uncertainty of realizing a benefit from those items. All of the Company’s operating losses since inception have been generated in the United States.

A summary of the Company’s current and deferred tax provision is as follows:


	Year ended December 31,
	2019		2018
Current income tax provision:
Federal	$	—	$	—
State		24		—
Total current income tax provision		24		—
Deferred income tax benefit:
Federal		(9,400)		(4,199)
State		688		(1,239)
Total deferred income tax benefit		(8,712)		(5,438)
Change in deferred tax asset valuation allowance		(8,712)		(5,438)
Total provision for income taxes	$	24	$	—

The $24 provision for income taxes for the year ended December 31, 2019 is classified within general and administrative expense on the Consolidated Statements of Operations and Comprehensive Loss.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:


	Year Ended December 31,
	2019		2018
Federal statutory income tax rate	21.0	%	21.0	%
State income taxes, net of federal benefit	(1.6)		1.5
Research and development tax credits	1.9		0.1
Change in preferred stock tranche obligation	-		(16.0)
Change in deferred tax asset valuation allowance	(19.9)		(6.6)
Effect of Section 382 limitation	(1.5)		-
Effective income tax rate	(0.1)	%	0.0	%

Net deferred tax assets as of December 31, 2019 and 2018 consisted of the following:


	December 31,
	2019		2018
Deferred tax assets
Net operating loss carry forwards	$	11,380	$	2,899
Research and development tax credit carry forwards		969		231
License fees		3,232		3,669
Stock based compensation		196		—
Accruals, reserves and other		44		1
Total deferred tax assets		15,821		6,800
Deferred tax liabilities
Prepaid expenses		(309)		—
Total deferred tax liabilities		(309)		—
Net deferred tax assets		15,512		6,800
Valuation allowance		(15,512)		(6,800)
Total net deferred tax assets	$	—	$	—

As of December 31, 2019, the Company had U.S. federal and state net operating loss carryforwards of $51,094 and $10,222, respectively, which may be available to offset future taxable income and begin to expire in 2037. The federal net operating loss carryforwards include $48,757, which may be carried forward indefinitely. As of December 31, 2019, the Company also had U.S. federal and state research and development tax credit carryforwards of $1,058 and $187, respectively, which may be available to offset future tax liabilities and begin to expire in 2032. During the year ended December 31, 2019, gross deferred tax assets, before valuation allowance, increased by $8,712, due primarily to the operating loss incurred by the Company during that period.

Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Sections 382 and 383 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income or tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three‑year period. The annual limitation is determined by multiplying the value of the Company's stock at the time of such ownership change by the applicable long-term tax-exempt rate. Such limitations may result in expiration of a portion of the NOL carryforwards before utilization. As of December 31, 2019, the Company determined that ownership changes occurred on March 24, 2017 and June 7, 2018. As a result of the ownership changes, approximately $2.2 million and $3.7 million of the NOLs will expire unutilized for federal and state purposes, respectively. The ability of the Company to use its remaining NOL carryforwards may be further limited if the Company experiences a Section 382 ownership change as a result of future changes in its stock ownership.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize its deferred tax assets at each reporting period. In doing so, the Company has considered its history of cumulative net losses incurred and its lack of commercialization of any products or generation of any revenue from product sales and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been recorded against the net deferred tax assets as of December 31, 2019 and 2018.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2019 and 2018 related primarily to increases in net operating loss carryforwards and research and development tax credit carryforwards, as follows:


	2019		2018
Valuation allowance as of January 1,	$	(6,800)	$	(1,362)
Increases recorded to income tax provision		—		—
Decreases recorded as a benefit to income tax provision		(8,712)		(5,438)
Valuation allowance as of December 31,	$	(15,512)	$	(6,800)

As of December 31, 2019, the Company had gross unrecognized tax benefits of $237 which were derived during the preceding twelve months, none of which if recognized, would reduce the effective tax rate in a future period, due to the Company's full valuation allowance on U.S. net deferred tax assets. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision. As of December 31, 2019, the Company had not accrued interest or penalties related to uncertain tax positions and no amounts had been recognized in the Company’s consolidated statements of operations and comprehensive loss. For the year ended December 31, 2019, the Company will file income tax returns in the U.S., California, Connecticut, Massachusetts, Maryland, New York, and Pennsylvania, as prescribed by the tax laws of the jurisdictions in which it operates. The Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company is open to future tax examination under statute from 2017 to the present.

A reconciliation of the beginning and ending unrecognized tax benefits for the year ended December 31, 2019 is as follows


	2019
Balance as of December 31, 2018	$	—
Increases related to prior year tax positions		—
Decreases related to prior year tax positions		—
Increases related to current year tax positions		237
Settlements		—
Lapse of statute of limitations		—
Balance as of December 31, 2019	$	237

Net loss per share

12 Months Ended

Dec. 31, 2019

Net loss per share

Net loss per share

11. Net loss per share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:


	Year Ended December 31,
	2019		2018
Numerator:
Net loss	$	(43,755)	$	(81,714)
Accretion of redeemable convertible preferred stock to redemption value		—		(520)
Net loss attributable to common stockholders, basic and diluted	$	(43,755)	$	(82,234)
Denominator:
Weighted average common shares outstanding, basic and diluted		15,070,728		103,403
Net loss per share attributable to common stockholders, basic and diluted	$	(2.90)	$	(795.28)

The Company excluded 49,568 shares and 125,790 shares of restricted common stock, presented on a weighted average basis, from the calculations of basic net loss per share attributable to common stockholders for the years ended December 31, 2019 and 2018, respectively, because those shares had not vested.

The Company’s potentially dilutive securities, which include stock options, unvested restricted common stock and redeemable convertible preferred stock, have been excluded from the computation of diluted net loss per share attributable to common stockholders as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:


	Year Ended December 31,
	2019	2018

Options to purchase common stock	3,096,555	1,839,913
Unvested restricted common stock	155,149	80,190
Redeemable convertible preferred stock (as converted to common stock)	—	21,056,136
	3,251,704	22,976,239

Commitments and contingencies	12 Months Ended
Commitments and contingencies	Dec. 31, 2019
Commitments and contingencies
Commitments and contingencies	12. Commitments and contingencies Lease agreements The Company entered into a use and occupancy agreement for office space in Cambridge, Massachusetts on August 15, 2018, with Atlas Venture Life Science Advisors, LLC, a related party (See Note 13). The parties terminated the agreement in September 2019. In October 2018, the Company entered into a lease agreement for office space in South San Francisco, California. In March 2019, the Company amended this lease agreement (the “First Amendment”) to extend the term of the lease and expand the square footage of the existing leased office space. The First Amendment lease expires in March 2021. The Company provided a security deposit of $20, which is included as a component of other assets (non-current) on the Company’s consolidated balance sheets as of December 31, 2019 and 2018. In September 2019 the Company entered into an agreement to use office space in Cambridge, Massachusetts. The agreement is for an initial six-month term with rolling six-month extensions. The Company recognizes rent expense on a straight-line basis over the respective lease periods and has recorded rent expense of $305 and $12 for the years ended December 31, 2019 and 2018, respectively. As of December 31, 2019, total future minimum commitments due ender our leases are $401, of which $321 is due in 2020 and $80 is due in 2021. Research and manufacturing commitments The Company has entered into agreements with contract research organizations and contract manufacturing organizations to provide services in connection with its nonclinical studies and clinical trials and to manufacture clinical development materials. As of December 31, 2019, the Company had non-cancelable purchase commitments under these agreements totaling $4,358. Indemnification agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not currently aware of any indemnification claims and has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2019 or 2018. Legal proceedings The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company intends to expense as incurred the costs related to such legal proceedings if they should arise.

Related party transactions	12 Months Ended
Related party transactions	Dec. 31, 2019
Related party transactions
Related party transactions	13. Related party transactions Apple Tree Life Sciences, Inc. During 2018, the Company issued 8,000,000 shares of Series A Preferred Stock and 880,568 shares of Series B Preferred Stock to entities affiliated with Apple Tree Life Sciences, Inc. ("Apple Tree") and a principal of Apple Tree was elected to the board of directors. The Company's founders, including the current Executive Vice President and Chief Operating Officer and Chief Scientific Officer, were formerly employees of Apple Tree until April 2017. During the years ended December 31, 2019 and 2018, the Company incurred fees for certain general and administrative services from Apple Tree totaling $20 and $0, respectively. As of December 31, 2019 and 2018, the Company did not owe any amounts to Apple Tree. Atlas Venture Life Science Advisors, LLC During 2018, the Company issued 10,666,667 shares of Series A Preferred Stock and 722,737 shares of Series B Preferred Stock to entities affiliated with Atlas Venture ("Atlas") and a principal of Atlas was elected to the board of directors. During the years ended December 31, 2019 and 2018, the Company incurred fees for certain research and development consulting services from Atlas totaling $0 and $23, respectively. In August 2018, the Company entered into a use and occupancy agreement for office space in Cambridge, Massachusetts with Atlas (See Note 12). The parties terminated the agreement in September of 2019. During the years ended December 31, 2019 and 2018, the Company incurred fees under the use and occupancy agreement with Atlas totaling $22 and $8, respectively. As of December 31, 2019, the Company did not owe any amounts to Atlas. As of December 31, 2018, the Company owed $12 to Atlas, which was included in accounts payable on the Company’s consolidated balance sheet. Versant Venture Capital VI, L.P. During 2018, the Company issued 10,666,667 shares of Series A Preferred Stock and 722,737 shares of Series B Preferred Stock to entities affiliated with Versant Venture Capital VI, L.P. ("Versant") and a principal of Versant at that time was elected to the board of directors. During the years ended December 31, 2019 and 2018, the Company incurred fees for certain general and administrative services from Versant totaling $0 and $4, respectively. As of December 31, 2019 and 2018, the Company did not owe any amounts to Versant. venBio Global Strategic Fund II, L.P. During 2018, the Company issued 10,666,667 shares of Series A Preferred Stock and 722,737 shares of Series B Preferred Stock to entities affiliated with venBio Global Strategic Fund II, L.P. ("venBio"). A principal of venBio served on the Company’s board of directors from June 2018 to August 2019. During the years ended December 31, 2019 and 2018, the Company incurred fees for certain general and administrative services from venBio totaling $0 and $35, respectively. As of December 31, 2019, the Company did not owe any amounts to venBio. As of December 31, 2018, the Company owed $35 to venBio, which was included in accounts payable on the Company’s consolidated balance sheet.

Benefit plans	12 Months Ended
Benefit plans	Dec. 31, 2019
Benefit plans
Benefit plans	14. Benefit plans The Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre‑tax basis. Matching contributions to the plan may be made at the discretion of the Company’s board of directors. The Company did not make any matching contributions to the plan during the years ended December 31, 2019 and 2018, respectively.

Subsequent events	12 Months Ended
Subsequent events	Dec. 31, 2019
Subsequent events
Subsequent events	15. Subsequent events The Company evaluated subsequent events through March 16, 2020, the date on which these financial statements were issued. On February 14, 2020, the Company entered into a seven-year lease agreement for 6,647 square feet of office space in South San Francisco, California. Under the agreement, the Company is required to make $2.3 million in minimum payments during the lease term. The Company anticipates that it will begin occupancy and commence the lease on or about June 1, 2020.

Summary of significant accounting policies (Policies)	12 Months Ended
Summary of significant accounting policies (Policies)	Dec. 31, 2019
Summary of significant accounting policies
Use of estimates	Use of estimates The preparation of the Company's consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses and the valuations of common stock, preferred stock tranche obligation, anti-dilution right liability and the valuation allowance for deferred tax assets. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value.
Short-term marketable securities	Short-term marketable securities The Company invests in short-term marketable securities, primarily money market funds, commercial paper, U.S. treasury securities and corporate debt securities. The Company classifies its short-term marketable securities as available-for-sale securities and reports them at fair value in short-term marketable securities on the consolidated balance sheets with related unrealized losses included within accumulated other comprehensive loss on the consolidated balance sheets. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in other income (expense), net on the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income (expense), net. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in other income (expense), net. The Company regularly reviews all its investments for other-than-temporary declines in estimated fair value. This review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. When the Company determines that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, the carrying value of the security will be reduced and a loss will be recorded for the amount of such decline.
Restricted cash	Restricted cash As of December 31, 2019 and 2018, the Company was required to maintain separate cash balances of $40 and $20, respectively, to collateralize corporate credit cards with a bank, which are classified within other assets (non-current) on the consolidated balance sheets. As of December 31, 2019 the Company was required to maintain a separate cash balance of $20 for the benefit of the landlord in connection with the Company’s office space lease in South San Francisco, California (the “Lease”), which is classified within other assets (non-current) on the 2019 consolidated balance sheet (see Note 12). As of December 31, 2018, the Company was required to maintain a separate cash balance of $5 for the benefit of the landlord in connection with the Lease, which is classified within other current assets on the 2018 consolidated balance sheet.
Concentrations of credit risk	Concentrations of credit risk Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents and short-term marketable securities. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company deposits its cash investments in financial institutions that it believes have high credit quality and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships. At December 31, 2019 and 2018, all of the Company's cash, cash equivalents and short-term investments were held at one accredited financial institution.
Deferred offering costs	Deferred offering costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process preferred stock or common equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded as a reduction to the carrying value of redeemable convertible preferred stock or in stockholders' equity (deficit) as a reduction of additional paid-in capital generated as a result of such offering. Should an in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the consolidated statements of operations and comprehensive loss. As of December 31, 2019, the Company did not have any deferred offering costs. As of December 31, 2018, the Company recorded deferred offering costs of $361, which are classified within prepaid expenses and other current assets on the 2018 consolidated balance sheet.
Segment information	Segment information The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company's singular focus is developing and commercializing transformative treatments for serious metabolic diseases, with an initial focus on NASH.
Research and development costs	Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred to discover, research and develop drug candidates, including personnel expenses, stock-based compensation expense, third-party license fees and external costs including fees paid to consultants and clinical research organizations ("CROs"), in connection with drug product manufacturing, nonclinical studies and clinical trials, and other related clinical trial fees, such as for investigator grants, patient screening, laboratory work, clinical trial database management, clinical trial material management and statistical compilation and analysis. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered or the services rendered. Costs incurred in obtaining technology licenses are charged immediately to research and development expense if the technology licensed has not reached technological feasibility and has no alternative future uses.
Research contract costs and accruals	Research contract costs and accruals The Company has entered into various research and development and other agreements with commercial firms, researchers and others for provisions of goods and services. These agreements are generally cancelable, and the related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ materially from the Company's estimates.
Patent costs	Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-based compensation	Stock-based compensation The Company measures all stock-based awards granted to employees and nonemployees based on the fair value on the date of the grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award, on a straight-line basis. The Company accounts for forfeitures as they occur. The Company estimates the fair value of stock option grants using the Black-Scholes option pricing model. Prior to our initial public offering, the exercise price for all stock options granted was at the estimated fair value of the underlying common stock as determined on the date of grant by the Company’s board of directors. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model, which requires inputs based on certain subjective assumptions, including the expected stock price volatility, the expected term of the option, the risk-free interest rate for a period that approximates the expected term of the option, and the Company's expected dividend yield. The Company went public in June 2019 and accordingly, lacks sufficient company-specific historical and implied volatility information for its shares traded in the public markets. Therefore, it estimates its expected share price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The expected term of the Company's stock options has been determined utilizing the "simplified" method for awards that qualify as "plain-vanilla" options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. The fair value of each restricted common stock award is estimated on the date of grant based on the fair value of the Company's common stock on that same date. Compensation expense for purchases under the Employee Stock Purchase Plan is recognized based on the fair value of the common stock estimated based on the closing price of our common stock as reported on the date of offering, less the purchase discount percentage provided for in the plan. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient's payroll costs are classified or in which the award recipient's service payments are classified.
Preferred stock tranche obligation	Preferred stock tranche obligation The Company classified the preferred stock tranche obligation for the future purchase, and option to purchase, Series A Preferred Stock (see Note 6) as a liability on its consolidated balance sheets as the preferred stock tranche obligation was a freestanding financial instrument that required the Company to transfer equity instruments upon future closings of the Series A Preferred Stock. The preferred stock tranche obligation was initially recorded at fair value upon the date of issuance and was subsequently remeasured to fair value at each reporting date. Changes in the fair value of the preferred stock tranche obligation were recognized as a component of other expense in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the preferred stock tranche obligation were recognized until the tranche obligations were fulfilled or otherwise extinguished in the fourth quarter of 2018. In November 2018, in connection with the Company's issuance and sale of Series A Preferred Stock, the Company satisfied its obligation to issue additional shares under the Second Tranche Closing. In December 2018, in connection with the Company's issuance and sale of Series B Preferred Stock, the Company terminated the option to purchase Series A Preferred Stock provided under the 2018 Series A Agreement (see Notes 3 and 6).
Anti-dilution right liability	Anti-dilution right liability The Company classified the anti-dilution right under its license agreement with Amgen Inc. ("Amgen") (see Note 9) as a derivative liability on its consolidated balance sheets as the anti-dilution right represented a freestanding financial instrument that required the Company to transfer equity instruments upon future equity closings. The anti-dilution right liability was initially recorded at fair value upon the date of issuance and was subsequently remeasured to fair value at each reporting date. The issuance date fair value of the anti-dilution right liability was recognized as a research and development expense upon entering into the agreement with Amgen. Changes in the fair value of the anti-dilution right liability were recognized as a component of other expense in the consolidated statements of operations and comprehensive loss. Changes in the fair value of the anti-dilution right liability were recognized until the anti-dilution right was satisfied in the fourth quarter of 2018. In November 2018, in connection with the Company’s issuance and sale of Series A Preferred Stock, the Company satisfied its anti-dilution right under the Amgen Agreement (see Notes 6 and 9).
Classification and accretion of redeemable convertible preferred stock	Classification and accretion of redeemable convertible preferred stock The Company has classified its redeemable convertible preferred stock outside of stockholders' equity (deficit) because the shares contain certain redemption features that are not solely within the control of the Company. Costs incurred in connection with the issuance of redeemable convertible preferred stock, as well as the recognition of the preferred stock tranche obligation, are recorded as a reduction of gross proceeds from issuance. The net carrying value of redeemable convertible preferred stock were accreted to their redemption values through a charge to additional paid-in capital or accumulated deficit over the period from date of issuance to the earliest date on which the holders could, at their option, elect to redeem their shares. In December 2018, in connection with the Company's issuance and sale of Series B Preferred Stock, the Company terminated the redemption rights associated with the Series A Preferred Stock that allowed the holders, at their option, to elect to redeem their shares at a specified date. Accordingly, the Company ceased accreting the net carrying value of the Series A redeemable convertible preferred stock to the redemption value.
Comprehensive loss	Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders' equity (deficit) that result from transactions and economic events other than those with stockholders. Our comprehensive loss is comprised of net loss and changes in unrealized gains and losses on our short-term marketable securities.
Income taxes	Income taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company's tax returns. Deferred tax assets and liabilities are determined based on the difference between the consolidated financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Net loss per share	Net loss per share The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. For purpose of this calculation, unvested restricted common stock and redeemable convertible preferred stock are considered potential dilutive common shares. The Company’s redeemable convertible preferred stock contractually entitles the holders of such shares to participate in dividends but does not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 2019 and 2018.
Emerging growth company	Emerging growth company The Company is an emerging growth company as defined in the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”). Under the JOBS Act, companies have extended transition periods available for complying with new or revised accounting standards. The Company has elected this exemption to delay adopting new or revised accounting standards until such time as those standards apply.
Recently issued accounting pronouncements not yet adopted	Recently issued accounting pronouncements not yet adopted In February 2016, the FASB issued ASU No. 2016‑02, Leases (Topic 842), subsequently amended by ASU 2018-10, ASU 2018-11, ASU 2019-01 and ASU 2019-10 (collectively, “ASU 2016‑02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification on the consolidated balance sheets. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. The Company intends to utilize the modified retrospective approach to adopting ASU 2016‑02 effective January 1, 2020. Further, the Company intends to utilize the package of available practical expedients which allows it to i) not reassess whether any expired or existing contracts are or contain leases; ii) not reassess the lease classification for expired or existing leases; and iii) not reassess the treatment of initial direct costs for any existing leases. The Company is in the process of completing a review of its existing lease agreements under ASC 842 and does not expect that the impact of the adoption of ASU 2016-02 on its consolidated balance sheets will be material and does not expect the adoption to have a material impact on its results of operations or cash flows. In February 2020 the Company entered into a new office lease agreement (see Note 15). The Company is in the process of determining the impact on its consolidated financial statements of the adoption of ASU 2016-02 related to this new lease. In August 2018, the FASB issued No. ASU 2018‑13, Fair Value Measurement (Topic 820)—Disclosure Framework (“ASU 2018‑13”), which improves the disclosure requirements for fair value measurements. For non-public entities, ASU 2018‑13 is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted for any removed or modified disclosures. The Company is currently evaluating the impact that the adoption of ASU 2018‑13 will have on its consolidated financial statements.

Fair value of financial assets and liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Fair value of financial assets and liabilities

Summary of assets that are measured at fair value on a recurring basis


	Fair Value Measurements as of December 31, 2019 Using:
	Total		Level 1		Level 2		Level 3
Money market funds	$	49,948	$	49,948	$	—	$	—
Commercial paper		49,114		—		49,114		—
U.S. treasury securities		6,048		6,048		—		—
Corporate debt securities		20,143		—		20,143		—
	$	125,253	$	55,996	$	69,257	$	—

Schedule of roll forward of the fair values of the Company’s preferred stock tranche obligation and anti‑dilution right liability


	Preferred stock		Anti‑dilution
	tranche		right
	obligation		liability
Balance as of December 31, 2017	$	—	$	—
Initial fair value of anti‑dilution right liability in connection with Amgen license agreement		—		1,639
Initial fair value of preferred stock tranche obligation in connection with the issuance of Series A Preferred Stock		7,350		—
Change in fair value		62,150		5,765
Settlement of future purchase obligation		(32,750)		—
Settlement of anti‑dilution right liability upon issuance of Series A preferred stock		—		(7,404)
Extinguishment of call option liability		(36,750)		—
Balance as of December 31, 2018	$	—	$	—

Short-term marketable securities (Tables)

12 Months Ended

Dec. 31, 2019

Short-term marketable securities

Schedule of short-term marketable securities


	December 31, 2019
			Gross		Gross
	Amortized		unrealized		unrealized		Fair
	Cost		gains		losses		value
Money market funds	$	49,948	$	—	$	—	$	49,948
Commercial paper		49,114		—		—		49,114
U.S. treasury securities		6,048		—		—		6,048
Corporate debt securities		20,149		—		(6)		20,143
	$	125,259	$	—	$	(6)	$	125,253

Cash and cash equivalents							$	53,641
Short-term marketable securities								71,612
							$	125,253

Accrued expenses and other current liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Accrued expenses and other current liabilities

Schedule of accrued expenses and other current liabilities


	December 31,
	2019		2018
Accrued employee compensation and benefits	$	1,606	$	304
Accrued external research and development expenses		6,361		430
Accrued legal and professional fees		370		106
Other		85		129
	$	8,422	$	969

Redeemable convertible preferred stock (Tables)

12 Months Ended

Dec. 31, 2019

Redeemable convertible preferred stock

Schedule of redeemable convertible preferred stock


		Preferred
	Preferred	stock					Common stock
	stock	issued and	Carrying		Liquidation		issuable upon
	authorized	outstanding	value		preference		conversion
Series A Preferred Stock	50,858,462	50,858,462	$	79,457	$	50,858	16,543,739
Series B Preferred Stock	13,871,948	13,871,948	$	45,271	$	45,500	4,512,397
	64,730,410	64,730,410	$	124,728	$	96,358	21,056,136

Stockholder’s equity (deficit) (Tables)

12 Months Ended

Dec. 31, 2019

Stockholder’s equity (deficit)

Schedule of common stock reserved for issuance


	December 31,
	2019	2018
Conversion of outstanding shares of preferred stock	—	21,056,136
Options outstanding under the 2018 Stock Option and Grant Plan	2,296,029	1,839,913
Options outstanding under the 2019 Stock Option and Incentive Plan	800,526	—
Options available for future grant	1,771,931	1,219,461
2019 Employee Stock Purchase Plan	269,364	—
	5,137,850	24,115,510

Summary of restricted stock activity

The following table summarizes restricted stock activity since December 31, 2017:


		Grant-Date Fair
	Number of Shares	Value
Unvested restricted common stock as of December 31, 2017	226,400	$	—
Shares vesting	(146,210)		—
Unvested restricted common stock as of December 31, 2018	80,190		—
Early exercise of unvested stock options	491,207		0.65
Shares vesting	(416,248)		0.65
Unvested restricted common stock as of December 31, 2019	155,149	$	0.65

Stock-based awards (Tables)

12 Months Ended

Dec. 31, 2019

Stock-based awards

Schedule of assumptions used to determine grant-date fair value of stock options granted


	Year Ended December 31,
	2019			2018
Weighted average risk‑free interest rate		2.12	%		2.99	%
Expected term (in years)		6.0			6.0
Expected volatility		73.75	%		70.88	%
Expected dividend yield		0	%		0	%

Summary of stock option activity


		Weighted-		Weighted-
		Average		Average	Aggregate
		Exercise		remaining	Intrinsic
	Number	Price per		contractual	Value
	of Options	Share		term (years)	(000's)
Balance Outstanding, December 31, 2017	—	$	—	—	$	—
Options granted	1,883,067		0.61	9.74
Options exercised	(12,586)		0.61
Options cancelled	(30,568)		0.61
Balance Outstanding, December 31, 2018	1,839,913		0.61	9.74		10,577
Options granted	1,912,352		12.42
Options exercised	(655,710)		0.69
Balance Outstanding, December 31, 2019	3,096,555	$	7.89	9.19	$	44,323
Exercisable, December 31, 2019	175,300	$	3.54	8.88	$	3,270
Vested and expected to vest, December 31, 2019	3,096,555	$	7.89	9.19	$	44,323

Income taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income taxes

Schedule of current and deferred tax provision


	Year ended December 31,
	2019		2018
Current income tax provision:
Federal	$	—	$	—
State		24		—
Total current income tax provision		24		—
Deferred income tax benefit:
Federal		(9,400)		(4,199)
State		688		(1,239)
Total deferred income tax benefit		(8,712)		(5,438)
Change in deferred tax asset valuation allowance		(8,712)		(5,438)
Total provision for income taxes	$	24	$	—

Schedule of reconciliation of the U.S. federal statutory income tax


	Year Ended December 31,
	2019		2018
Federal statutory income tax rate	21.0	%	21.0	%
State income taxes, net of federal benefit	(1.6)		1.5
Research and development tax credits	1.9		0.1
Change in preferred stock tranche obligation	-		(16.0)
Change in deferred tax asset valuation allowance	(19.9)		(6.6)
Effect of Section 382 limitation	(1.5)		-
Effective income tax rate	(0.1)	%	0.0	%

Schedule of Net deferred tax assets


	December 31,
	2019		2018
Deferred tax assets
Net operating loss carry forwards	$	11,380	$	2,899
Research and development tax credit carry forwards		969		231
License fees		3,232		3,669
Stock based compensation		196		—
Accruals, reserves and other		44		1
Total deferred tax assets		15,821		6,800
Deferred tax liabilities
Prepaid expenses		(309)		—
Total deferred tax liabilities		(309)		—
Net deferred tax assets		15,512		6,800
Valuation allowance		(15,512)		(6,800)
Total net deferred tax assets	$	—	$	—

Schedule of changes in the valuation allowance for deferred tax assets


	2019		2018
Valuation allowance as of January 1,	$	(6,800)	$	(1,362)
Increases recorded to income tax provision		—		—
Decreases recorded as a benefit to income tax provision		(8,712)		(5,438)
Valuation allowance as of December 31,	$	(15,512)	$	(6,800)

Schedule of unrecognized tax benefits roll Forward


	2019
Balance as of December 31, 2018	$	—
Increases related to prior year tax positions		—
Decreases related to prior year tax positions		—
Increases related to current year tax positions		237
Settlements		—
Lapse of statute of limitations		—
Balance as of December 31, 2019	$	237

Net loss per share (Tables)

12 Months Ended

Dec. 31, 2019

Net loss per share

Schedule of basic and diluted net loss per share attributable to common stockholders


	Year Ended December 31,
	2019		2018
Numerator:
Net loss	$	(43,755)	$	(81,714)
Accretion of redeemable convertible preferred stock to redemption value		—		(520)
Net loss attributable to common stockholders, basic and diluted	$	(43,755)	$	(82,234)
Denominator:
Weighted average common shares outstanding, basic and diluted		15,070,728		103,403
Net loss per share attributable to common stockholders, basic and diluted	$	(2.90)	$	(795.28)

Schedule of computation of diluted net loss per share attributable to common stockholders


	Year Ended December 31,
	2019	2018

Options to purchase common stock	3,096,555	1,839,913
Unvested restricted common stock	155,149	80,190
Redeemable convertible preferred stock (as converted to common stock)	—	21,056,136
	3,251,704	22,976,239

Nature of the business and basis of presentation (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 24, 2019 USD ($) $ / shares shares	Jun. 06, 2019	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Subsidiary, Sale of Stock
Common stock issued (in shares) \| shares			28,567,837	238,986
Proceeds received, net of underwriting discounts and commissions			$ 85
Offering costs				$ 449
Authorized common stock (in shares) \| shares			150,000,000	75,000,000
Par value common stock (in dollars per share) \| $ / shares			$ 0.0001	$ 0.0001
Reverse stock split of the common stock		3.07418
Net loss			$ 43,755	$ 81,714
Accumulated deficit			130,320	$ 86,565
Cash, cash equivalents and short-term marketable securities			$ 136,400
IPO
Subsidiary, Sale of Stock
Common stock issued (in shares) \| shares	6,612,500
Public offering price (in dollars per share) \| $ / shares	$ 16.00
Proceeds received, net of underwriting discounts and commissions	$ 98,394
Offering costs	$ 2,942
Conversion of convertible preferred stock into common stock \| shares	21,056,136
Authorized common stock (in shares) \| shares	150,000,000
Par value common stock (in dollars per share) \| $ / shares	$ 0.0001
Authorized Preferred stock (in shares) \| shares	10,000,000
Par value preferred stock (in dollars per share) \| $ / shares	$ 0.0001
Over allotment
Subsidiary, Sale of Stock
Proceeds received, net of underwriting discounts and commissions	$ 98,394
Over allotment \| Maximum
Subsidiary, Sale of Stock
Common stock issued (in shares) \| shares	862,500

Summary of significant accounting policies - Restricted cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Summary of significant accounting policies
Separate cash balance to collateralize corporate credit cards with a bank	$ 40	$ 20
Separate cash balance for the landlord	$ 20	$ 5

Summary of significant accounting policies - Deferred offering costs (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Summary of significant accounting policies
Deferred offering costs	$ 0	$ 361

Summary of significant accounting policies - Recently issue accounting Pronouncement (Details) $ in Thousands	Jan. 01, 2020 USD ($)
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-Use asset	$ 300

Fair value of financial assets and liabilities - Recurring (Details) - Recurring $ in Thousands	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 125,253
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	49,948
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	49,114
U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	6,048
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	20,143
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	55,996
Level 1 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	49,948
Level 1 \| U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	6,048
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	69,257
Level 2 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	49,114
Level 2 \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 20,143

Fair value of financial assets and liabilities - Textual (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Nov. 30, 2018 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair value of financial assets and liabilities
Settlement of future purchase obligation				$ (32,750)
Initial fair value of the anti-dilution right liability				1,639
Gain on extinguishment recognized	$ 36,750			(36,750)
Transfers from level 1 to level 2, Assets	0		$ 0	0
Transfers from level 2 to level 1, Assets	0		0	0
Transfers from level 1 to level 2, Liabilities	0		0	0
Transfers from level 2 to level 1, Liabilities	$ 0		0	0
Transfers into level 3 Assets			0	0
Transfers out of level 3 Assets			0	0
Transfers into level 3 liabilities			0	0
Transfers out of level 3 liabilities			$ 0	$ 0
Expected Dividend Rate
Fair value of financial assets and liabilities
Expected dividend yield			0
Amgen Agreement
Fair value of financial assets and liabilities
Initial fair value of the anti-dilution right liability		$ 7,404
Series A Preferred Stock \| Amgen Agreement
Fair value of financial assets and liabilities
Share Issued under agreement \| shares		3,205,128
Second Tranche Closing
Fair value of financial assets and liabilities
Settlement of future purchase obligation		$ 32,750
Second Tranche Closing \| Series A Preferred Stock \| Amgen Agreement
Fair value of financial assets and liabilities
Share Issued under agreement \| shares		3,205,128
Initial fair value of the anti-dilution right liability		$ 7,404

Fair value of financial assets and liabilities - Rollforward (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018
Fair value of financial assets and liabilities
Initial fair value of anti‑dilution right liability in connection with Amgen license agreement		$ 1,639
Initial fair value of preferred stock tranche obligation in connection with the issuance of Series A Preferred Stock		7,350
Change in fair value of preferred stock tranche liability		62,150
Change in fair value of anti-dilution right liability		5,765
Settlement of future purchase obligation		(32,750)
Settlement of anti‑dilution right liability upon issuance of Series A preferred stock		(7,404)
Extinguishment of call option liability	$ 36,750	$ (36,750)

Short-term marketable securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 125,259
Gross unrealized losses	(6)
Fair value	125,253
Cash and cash equivalents	53,641
Short-term marketable securities	71,612	$ 0
Total	125,253
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	49,948
Fair value	49,948
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	49,114
Fair value	49,114
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	6,048
Fair value	6,048
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	20,149
Gross unrealized losses	(6)
Fair value	$ 20,143

Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 1,606	$ 304
Accrued external research and development expenses	6,361	430
Accrued legal and professional fees	370	106
Other	85	129
Total accrued expenses and other current liabilities	$ 8,422	$ 969

Redeemable convertible preferred stock - Textual (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			12 Months Ended
	Jun. 24, 2019	Dec. 31, 2018	Nov. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity
Preferred Stock , shares issued		64,730,410			0	64,730,410
Preferred Stock , shares outstanding		64,730,410			0	64,730,410
Stock issuance cost						$ 449
Share Issued value under agreement					$ 95,453
Initial fair value of preferred stock tranche obligation in connection with the issuance of Series A Preferred Stock						7,350
Initial fair value of anti‑dilution right liability in connection with Amgen license agreement						$ 1,639
IPO
Temporary Equity
Preferred Stock , shares outstanding					0
Number of shares converted	21,056,136
Stock issuance cost	$ 2,942
Amgen Agreement
Temporary Equity
Initial fair value of anti‑dilution right liability in connection with Amgen license agreement			$ 7,404
Amgen Agreement \| Amgen
Temporary Equity
Share Issued under agreement				2,653,333
Share Issued value under agreement				$ 1,353
Redeemable Convertible Preferred Stock
Temporary Equity
Number of shares converted	21,056,136				(64,730,410)
Beneficial conversion feature					$ 0
Series A Preferred Stock
Temporary Equity
Share Issued under agreement						17,653,333
Aggregate proceeds				14,784
Stock issuance cost				$ 216
Share Issued value under agreement						$ 8,787
Net Issuance cost						$ 216
Series B Preferred Stock
Temporary Equity
Share Issued under agreement		13,871,948				13,871,948
Original Issue price (in dollars per share)		$ 3.28				$ 3.28
Aggregate proceeds		$ 45,500
Share Issued value under agreement						$ 45,271
Net Issuance cost		$ 229				$ 229
First Tranche Closing \| Series A Preferred Stock
Temporary Equity
Share Issued under agreement				15,000,000
Original Issue price (in dollars per share)				$ 1.00
Aggregate proceeds				$ 15,000
Preferred stock tranche obligation in connection with the issuance of Series A Preferred Stock					$ 7,350
Initial carrying value of stock issued				8,787
First Tranche Closing \| Series A Preferred Stock \| Amgen
Temporary Equity
Fair value of shares of Series A Preferred Stock issued to Amgen				1,353
Net Issuance cost				216
Initial fair value of preferred stock tranche obligation in connection with the issuance of Series A Preferred Stock				$ 7,350
Second Tranche Closing \| Amgen Agreement \| Amgen \| Akero Therapeutics, Inc.
Temporary Equity
Ownership interest of the outstanding shares (as a percent)					10.00%
Second Tranche Closing \| Series A Preferred Stock
Temporary Equity
Share Issued under agreement			25,000,001			25,000,001
Original Issue price (in dollars per share)			$ 1.00
Share Issued value under agreement						$ 24,996
Net Issuance cost			$ 4			$ 4
Preferred stock tranche obligation in associated with future purchase obligation			$ 32,750
Second Tranche Closing \| Series A Preferred Stock \| Amgen Agreement \| Amgen
Temporary Equity
Share Issued under agreement			3,205,128
Initial fair value of anti‑dilution right liability in connection with Amgen license agreement			$ 7,404
Period prior to January 14, 2022
Temporary Equity
Share Issued under agreement					20,000

Redeemable convertible preferred stock - Convertible Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019
Temporary Equity
Redeemable convertible preferred stock, shares issued	64,730,410	0
Preferred stock authorized	64,730,410	0
Preferred stock issued and outstanding	64,730,410
Carrying value	$ 124,728
Liquidation preference	$ 96,358
Common stock issuable upon conversion	21,056,136
Series A Preferred Stock
Temporary Equity
Preferred stock authorized	50,858,462
Preferred stock issued and outstanding	50,858,462
Carrying value	$ 79,457
Liquidation preference	$ 50,858
Common stock issuable upon conversion	16,543,739
Series B Preferred Stock
Temporary Equity
Preferred stock authorized	13,871,948
Preferred stock issued and outstanding	13,871,948
Carrying value	$ 45,271
Liquidation preference	$ 45,500
Common stock issuable upon conversion	4,512,397

Stockholder’s equity (deficit) - Common stock (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Jun. 24, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) Vote $ / shares shares	Jun. 18, 2019 shares	Dec. 31, 2018 $ / shares shares
Class of Stock
Common stock, shares authorized		150,000,000		75,000,000
Common stock, par value (in dollars per share) \| $ / shares		$ 0.0001		$ 0.0001
Number of votes per share \| Vote		1
Cash dividend declared \| $ / shares		$ 0
Cash dividend paid \| $ / shares		$ 0
Common stock, shares issued		28,567,837		238,986
Proceeds received, net of underwriting discounts and commissions \| $		$ 85
Common stock, shares outstanding		28,567,837		238,986
Common stock were reserved for issuance
Options available for future grant		5,137,850		24,115,510
Options to purchase common stock
Common stock were reserved for issuance
Options available for future grant		1,771,931		1,219,461
2018 Stock option and grant plan
Common stock were reserved for issuance
Options available for future grant		2,296,029	107,635	1,839,913
2019 Stock Option and Incentive plan
Common stock were reserved for issuance
Options available for future grant		800,526
2019 Employee Stock Purchase Plan
Common stock were reserved for issuance
Options available for future grant		269,364
Redeemable Convertible Preferred Stock
Class of Stock
Number of shares converted	21,056,136	(64,730,410)
Common stock were reserved for issuance
Options available for future grant				21,056,136
IPO
Class of Stock
Common stock, shares authorized	150,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001
Common stock, shares issued	6,612,500
Proceeds received, net of underwriting discounts and commissions \| $	$ 98,394
Number of shares converted	21,056,136
Over allotment
Class of Stock
Public offering price \| $ / shares	$ 16.00
Proceeds received, net of underwriting discounts and commissions \| $	$ 98,394
Offering costs \| $	$ 2,942
Over allotment \| Maximum
Class of Stock
Common stock, shares issued	862,500

Stockholder’s equity (deficit) - Undesignated preferred stock and Restricted common stock (Details) - $ / shares	1 Months Ended
	Oct. 31, 2017	Mar. 31, 2017	Dec. 31, 2019
Restricted stock
Class of Stock
Shares issued (in shares)		226,400
Stock options vesting period	2 years	4 years
Number of shares subject to be repurchased	75,467
Undesignated Preferred Stock
Class of Stock
Preferred stock, par value (in dollars per share)			$ 0.0001
Preferred stock, Shares Issued			0
Preferred stock, Shares Outstanding			0
Undesignated Preferred Stock \| Maximum
Class of Stock
Preferred stock, shares authorized			10,000,000

Stockholder's equity (deficit) - Restricted stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Beginning balance of unvested restricted common stock	80,190	226,400
Early exercise of unvested stock options	491,207
Vested	(416,248)	(146,210)
Ending balance of unvested restricted common stock	155,149	80,190
Grant-Date Fair Value
Early exercise of unvested stock options	$ 650.0000
Vested	650.0000
Ending balance of unvested restricted common stock	$ 650.0000

Stockholder's equity (deficit) - Restricted stock activity narrative (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Unvested restricted common stock	155,149	80,190	226,400
Early exercise of unvested stock options	491,207
Restricted stock
Number of Shares
Unvested restricted common stock	155,149
Early exercise of unvested stock options	131,551
Restricted stock \| Founders
Number of Shares
Unvested restricted common stock	23,598

Stock-based awards - Textual (Details) - shares		1 Months Ended
Stock-based awards - Textual (Details) - shares	Jun. 18, 2019	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based awards
Number of shares remained available for grant			5,137,850	24,115,510
2018 Stock option and grant plan
Share-based awards
Number of shares of common stock issued	3,071,960
Number of shares remained available for grant	107,635		2,296,029	1,839,913
Stock Options And Grant Plan 2019
Share-based awards
Number of shares remained available for grant	107,635
Number of shares initially reserved for issuance		2,572,457
Percent of shares reserved and available for issuance automatically increase annually on January 1		4.00%
Percentage of exercise price per share of stock options		100.00%
Percentage of minimum exercise price, holding more than 10% of voting power		110.00%
Stock options vesting period		4 years
2019 Employee stock purchase plan
Share-based awards
Number of shares initially reserved for issuance	273,869
Percent of shares reserved and available for issuance automatically increase annually on January 1		1.00%
Number of shares reserved and available for issuance automatically increase annually on January 1 (in shares)		410,803
Maximum \| Stock Options And Grant Plan 2019
Share-based awards
Term of stock option		10 years

Stock-based awards - Stock option valuation (Details)	12 Months Ended
Stock-based awards - Stock option valuation (Details)	Dec. 31, 2019	Dec. 31, 2018
Assumptions to determine grant-date fair value of stock options granted
Weighted average risk‑free interest rate	2.12%	2.99%
Expected term (in years)	6 years	6 years
Expected volatility	73.75%	70.88%
Expected dividend yield	0.00%	0.00%

Stock based awards - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Options
Balance Outstanding, Beginning of the period (in shares)	1,839,913
Options granted ( in shares)	1,912,352	1,883,067
Options exercised ( in shares)	(655,710)	(12,586)
Options cancelled (in shares)		(30,568)
Balance Outstanding, End of the period (in shares)	3,096,555	1,839,913
Exercisable ( in shares)	175,300
Vested and expected to vest ( in shares)	3,096,555
Weighted-Average Exercise Price per Share
Weighted-average exercise price, Beginning of the period (in dollars per share)	$ 0.61
Weighted-average exercise price, Granted (in dollars per share)	12.42	$ 0.61
Weighted-average exercise price, Exercised (in dollars per share)	0.69	0.61
Weighted-Average Exercise, cancelled (in dollars per share)		0.61
Weighted-average exercise price, End of the period (in dollars per share)	7.89	$ 0.61
Weighted-average exercise price, Exercisable ( in dollars per share )	3.54
Weighted-average exercise price, Vested and expected to vest at end of the period (in dollars per share)	$ 7.89
Weighted-Average remaining contractual term (years) and Aggregate Intrinsic Value
Weighted-average remaining contractual term outstanding (in years)	9 years 2 months 9 days	9 years 8 months 27 days
Weighted-average remaining contractual term , Options Exercisable(in years)	8 years 10 months 17 days	9 years 8 months 27 days
Weighted-average remaining contractual term vested and expected to vest (in years)	9 years 2 months 9 days
Aggregate Intrinsic Value Balance outstanding	$ 44,323	$ 10,577
Aggregate Intrinsic Value Exercisable	3,270
Aggregate Intrinsic Value Vested and Expected to vest	$ 44,323
Weighted average grant-date fair value per share of stock options granted	$ 8.34

Stock based awards - Stock-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Apr. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Stock-based compensation
Total stock based compensation for options granted				$ 1,770	$ 121
Total unrecognized compensation cost related to unvested stock-based awards				$ 15,584
Weighted average period of recognition for unrecognized compensation cost				3 years 29 days
Proceeds from the early exercise of stock options in exchange for restricted common stock				$ 321
Common Stock
Stock-based compensation
Issuance of restricted common stock upon early exercise of stock options (in shares)	491,207	491,207	491,207	491,207
Exercise price of common stock purchase	$ 0.65	$ 0.65	$ 0.65
Proceeds from the early exercise of stock options in exchange for restricted common stock	$ 321	$ 321	$ 321
Research and Development Expense
Stock-based compensation
Total stock based compensation for options granted				$ 485	41
General and Administrative Expense
Stock-based compensation
Total stock based compensation for options granted				$ 1,285	$ 80

Amgen license agreement (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
Amgen license agreement (Details) - USD ($) $ in Thousands	Nov. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Share Issued value under agreement			$ 95,453
Initial fair value of the anti-dilution right liability				$ 1,639
Loss recognized related to change in fair value of the anti-dilution right liability prior to extinguishment				(5,765)
Research and development expense			37,046	11,882
Upfront cash payment				5,000
Fair value of issuance of the anti-dilution right liability				1,639
Amgen
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payment for clinical trail			2,500
Amgen Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Initial fair value of the anti-dilution right liability	$ 7,404
Amgen Agreement \| Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate milestone payments upon the achievement of specified clinical and regulatory milestones			37,500
Aggregate milestone payments upon the achievement of specified commercial milestones			75,000
Amgen Agreement \| Amgen
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Ownership interest of the outstanding shares (as a percent)		10.00%
Amgen Agreement \| Research and Development Expense
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Initial fair value of the anti-dilution right liability		$ 1,639
Research and development expense				8,016
Research and development expense upon achievement of clinical milestone			$ 2,500
Upfront cash payment				5,000
Fair value of shares of Series A Preferred Stock issued to Amgen				1,353
Fair value of issuance of the anti-dilution right liability				1,639
Other research and development expenses				24
Amgen Agreement \| Other expense
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Loss recognized related to change in fair value of the anti-dilution right liability prior to extinguishment				$ 5,765
Amgen Agreement \| Amgen
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payment		$ 5,000
Share Issued under agreement		2,653,333
Share Issued value under agreement		$ 1,353
Total consideration transferred to Amgen		$ 6,353
Amgen Agreement \| Series A Preferred Stock
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Share Issued under agreement	3,205,128
Amgen Agreement \| Series A Preferred Stock \| Second Tranche Closing
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Share Issued under agreement	3,205,128
Initial fair value of the anti-dilution right liability	$ 7,404
Amgen Agreement \| Series A Preferred Stock \| Amgen
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Share Issued under agreement		2,653,333
Share Issued value under agreement		$ 1,353

Income taxes - Current income tax provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Current income tax provision:
Income tax benefit related to operating losses and research and development tax credits	$ 0	$ 0
State	24
Total current income tax provision	24
Deferred income tax benefit:
Federal	(9,400)	(4,199)
State	688	(1,239)
Total deferred income tax benefit	(8,712)	(5,438)
Change in deferred tax asset valuation allowance	(8,712)	$ (5,438)
Total provision for income taxes	$ 24

Income taxes - Income tax rate (Details)	12 Months Ended
Income taxes - Income tax rate (Details)	Dec. 31, 2019	Dec. 31, 2018
Income taxes
Federal statutory income tax rate	21.00%	21.00%
State income taxes, net of federal benefit	(1.60%)	1.50%
Research and development tax credits	1.90%	0.10%
Change in preferred stock tranche obligation		(16.00%)
Change in deferred tax asset valuation allowance	(19.90%)	(6.60%)
Other	(1.50%)
Effective income tax rate	(0.10%)	0.00%

Income taxes - Deferred tax (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax assets
Net operating loss carry forwards	$ 11,380	$ 2,899
Research and development tax credit carry forwards	969	231
License fees	3,232	3,669
Stock based compensation	196
Accruals, reserves and other	44	1
Total deferred tax assets	15,821	6,800
Defered tax liabilites
Prepaid expenses	(309)
Total deferred tax liabilities	(309)
Net deferred tax assets	15,512	6,800
Valuation allowance	$ (15,512)	$ (6,800)	$ (1,362)

Income taxes - Valuation allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income taxes
Valuation allowance as of January 1,	$ (6,800)	$ (1,362)
Decreases recorded as a benefit to income tax provision	(8,712)	(5,438)
Valuation allowance as of December 31,	$ (15,512)	$ (6,800)

Income taxes - Unrecognized tax benefits (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Increases related to current year tax positions	$ 237
Balance at the ending	$ 237

Income taxes - Textual (Details) - USD ($) $ in Thousands	12 Months Ended
Income taxes - Textual (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ 24
Net operating loss carry forwards	11,380	$ 2,899
Deferred Tax Assets, Operating Loss Carryforwards, Not Subject to Expiration	48,757
Deferred Tax Assets, Tax Credit Carryforwards, Research	969	231
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	(8,712)	$ (5,438)
Accrued interest or penalties related to uncertain tax positions	0
Gross unrecognized tax benefit	237
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	51,094
Deferred Tax Assets, Tax Credit Carryforwards, Research	1,058
NOL carryforwards that expire	2,200
State
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	10,222
Deferred Tax Assets, Tax Credit Carryforwards, Research	187
NOL carryforwards that expire	3,700
General and Administrative Expense
Operating Loss Carryforwards [Line Items]
Income tax benefits	$ 24

Net loss per share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (43,755)	$ (81,714)
Accretion of redeemable convertible preferred stock to redemption value		(520)
Net loss attributable to common stockholders, basic and diluted	$ (43,755)	$ (82,234)
Denominator:
Weighted average common shares outstanding, basic and diluted	15,070,728	103,403
Net loss per share attributable to common stockholders, basic and diluted	$ (2.90)	$ (795.28)
Number of weighted average restricted common stock	125,790	49,568,000

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net loss per share - Anti-Diluted (Details) - shares	12 Months Ended
Net loss per share - Anti-Diluted (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total	3,251,704	22,976,239
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total	3,096,555	1,839,913
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total	155,149	80,190
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares excluded from the calculation of diluted earnings per share, total		21,056,136

Commitments and contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Commitments and contingencies
Security deposit	$ 20	$ 20
Initial lease term under the agreement	6 months
Extension lease term under the agreement	6 months
Rent expense	$ 305
Rent expense		$ 12
Total future minimum commitments	401
2020	321
2021	80
Non-cancelable purchase commitments with contract research organizations and contract manufacturing organizations	$ 4,358

Related party transactions (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction
Share Issued value under agreement		$ 95,453
Apple Tree
Related Party Transaction
Expenses for services received from related party		20	$ 0
Versant
Related Party Transaction
Expenses for services received from related party		0	4
venBio
Related Party Transaction
Expenses for services received from related party		0	35
Due to related party	$ 35		$ 35
Series A Preferred Stock
Related Party Transaction
Issuance of stock (in shares)			17,653,333
Share Issued value under agreement			$ 8,787
Series A Preferred Stock \| Apple Tree
Related Party Transaction
Issuance of stock (in shares)			8,000,000
Series A Preferred Stock \| Versant
Related Party Transaction
Issuance of stock (in shares)			10,666,667
Series A Preferred Stock \| venBio
Related Party Transaction
Issuance of stock (in shares)			10,666,667
Series B Preferred Stock
Related Party Transaction
Issuance of stock (in shares)	13,871,948		13,871,948
Share Issued value under agreement			$ 45,271
Series B Preferred Stock \| Apple Tree
Related Party Transaction
Share Issued value under agreement			880,568
Series B Preferred Stock \| Versant
Related Party Transaction
Share Issued value under agreement			722,737
Series B Preferred Stock \| venBio
Related Party Transaction
Share Issued value under agreement			722,737
Atlas Venture Life Science Advisors, LLC
Related Party Transaction
Expenses for services received from related party		22	8
Amount owed to related party			$ 12
Atlas Venture Life Science Advisors, LLC \| Series A Preferred Stock
Related Party Transaction
Issuance of stock (in shares)			10,666,667
Atlas Venture Life Science Advisors, LLC \| Series B Preferred Stock
Related Party Transaction
Share Issued value under agreement			$ 722,737
Atlas Venture Life Science Advisors, LLC \| Research And Development Consulting Services
Related Party Transaction
Expenses for services received from related party		$ 0	$ 23

Subsequent events (Details) $ in Thousands	Feb. 14, 2020 USD ($) ft²	Dec. 31, 2019 USD ($)
Subsequent events
Minimum payments during the lease term		$ 401
Subsequent events
Subsequent events
Lease agreement term	7 years
Area of office space \| ft²	6,647
Minimum payments during the lease term	$ 2,300