Fulcrum Therapeutics (FULC) 10-Q 13 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 08, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Title of 12(b) Security	Common stock, par value $0.001 per share
Trading Symbol	FULC
Security Exchange Name	NASDAQ
Entity Registrant Name	FULCRUM THERAPEUTICS, INC.
Entity Central Index Key	0001680581
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		23,373,212
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity File Number	001-38978
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4839948
Entity Address, Address Line One	26 Landsdowne Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	651-8851
Document Quarterly Report	true
Document Transition Report	false

Consolidated Balance Sheets (Un

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 25,379	$ 96,713
Marketable securities	55,828
Unbilled accounts receivable	325
Prepaid expenses and other current assets	2,791	3,370
Total current assets	84,323	100,083
Property and equipment, net	9,362	9,205
Restricted cash	1,092	1,092
Other assets	79	59
Total assets	94,856	110,439
Current liabilities:
Accounts payable	3,937	2,186
Accrued expenses and other current liabilities	5,219	5,496
Deferred lease incentive, current portion	469	469
Deferred revenue, current portion	4,557	3,989
Total current liabilities	14,182	12,140
Deferred rent, excluding current portion	1,590	1,559
Deferred lease incentive, excluding current portion	3,404	3,521
Deferred revenue, excluding current portion	5,018	6,011
Other liabilities, excluding current portion	31	55
Total liabilities	24,225	23,286
Commitments and contingencies (Note 12)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 200,000,000 shares authorized; 23,363,199 and 23,335,514 shares issued as of March 31, 2020 and December 31, 2019, respectively; 22,793,137 and 22,654,444 shares outstanding as of March 31, 2020 and December 31, 2019, respectively	23	23
Additional paid-in capital	239,914	237,931
Accumulated other comprehensive loss	(53)
Accumulated deficit	(169,253)	(150,801)
Total stockholders’ equity	70,631	87,153
Total liabilities and stockholders’ equity	$ 94,856	$ 110,439

Consolidated Balance Sheets (_2

Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	23,363,199	23,335,514
Common stock, shares outstanding	22,793,137	22,654,444
Treasury stock, shares	0	0

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 750
Operating expenses:
Research and development	14,482	$ 34,629
General and administrative	5,064	2,598
Total operating expenses	19,546	37,227
Loss from operations	(18,796)	(37,227)
Other income, net	344	384
Net loss	(18,452)	(36,843)
Cumulative convertible preferred stock dividends		(3,041)
Net loss attributable to common stockholders	$ (18,452)	$ (39,884)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.81)	$ (24.29)
Weighted average number of common shares used in net loss per share attributable to common stockholders, basic and diluted	22,719	1,642
Comprehensive loss:
Net loss	$ (18,452)	$ (36,843)
Other comprehensive loss:
Unrealized losses on marketable securities	(53)
Total other comprehensive loss	(53)
Comprehensive loss	$ (18,505)	$ (36,843)

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income Loss	Accumulated Deficit
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2018		60,000,000	40,000,000
Convertible preferred stock, Beginning balance at Dec. 31, 2018		$ 59,909	$ 79,761
Beginning balance at Dec. 31, 2018	$ (63,670)			$ 2		$ 4,452		$ (68,124)
Beginning balance, Shares at Dec. 31, 2018				1,587,953	67,024
Convertible preferred stock, Ending balance, Shares at Mar. 31, 2019		60,000,000	52,500,000
Convertible preferred stock, Ending balance at Mar. 31, 2019		$ 59,909	$ 105,227
Issuance of convertible preferred stock			$ 25,466
Issuance of convertible preferred stock, Shares			12,500,000
Issuance of common stock	5					5
Issuance of common stock, Shares				134,013
Repurchase of unvested restricted stock awards					43,922
Retirement of treasury shares					(110,946)
Stock-based compensation expense	821					821
Net loss	(36,843)							(36,843)
Ending balance at Mar. 31, 2019	(99,687)			$ 2		5,278		(104,967)
Ending balance, Shares at Mar. 31, 2019				1,721,966
Beginning balance at Dec. 31, 2019	87,153			$ 23		237,931		(150,801)
Beginning balance, Shares at Dec. 31, 2019				22,654,444
Issuance of common stock	290					290
Issuance of common stock, Shares				138,693
Repurchase of unvested restricted stock awards					8,787
Retirement of treasury shares					(8,787)
Stock-based compensation expense	1,693					1,693
Unrealized loss on marketable securities	(53)						$ (53)
Net loss	(18,452)							(18,452)
Ending balance at Mar. 31, 2020	$ 70,631			$ 23		$ 239,914	$ (53)	$ (169,253)
Ending balance, Shares at Mar. 31, 2020				22,793,137

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities
Net loss	$ (18,452)	$ (36,843)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	539	504
Stock-based compensation expense	1,693	821
In-process research and development expenses		25,591
Net amortization of premiums and discounts on marketable securities	(45)
Changes in operating assets and liabilities:
Unbilled accounts receivable	(325)
Prepaid expenses and other current assets	579	(264)
Other assets	(20)	(63)
Accounts payable	1,429	1,233
Accrued expenses and other liabilities	(91)	(807)
Deferred revenue	(425)
Deferred rent and deferred lease incentive	(86)	(69)
Net cash used in operating activities	(15,204)	(9,897)
Investing activities
Purchases of marketable securities	(55,837)
Maturities of marketable securities	0
Purchases of property and equipment	(567)	(310)
Transaction costs associated with asset acquisition		(91)
Net cash used in investing activities	(56,404)	(401)
Financing activities
Issuance costs associated with issuance of Series B convertible preferred stock		(34)
Principal payments on capital lease obligations	(12)	(11)
Proceeds from issuance of common stock under benefit plans, net	286	(2)
Net cash provided by (used in) financing activities	274	(47)
Net decrease in cash, cash equivalents and restricted cash	(71,334)	(10,345)
Cash, cash equivalents, and restricted cash, beginning of period	97,805	73,889
Cash, cash equivalents, and restricted cash, end of period	26,471	63,544
Supplemental cash flow information
Cash paid for interest	1	2
Non-cash investing and financing activities:
Acquisition of in process research and development through issuance of stock		25,500
Property and equipment purchases unpaid at end of period	163	47
Public offering costs unpaid at end of period	301	190
Cash and cash equivalents	25,379	62,452
Restricted cash	$ 1,092	$ 1,092

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Fulcrum Therapeutics, Inc. (the “Company” or “Fulcrum”) was incorporated in Delaware on August 18, 2015. The Company is focused on improving the lives of patients with genetically defined rare diseases in areas of high unmet medical need. The Company is subject to a number of risks similar to other companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, dependence on key personnel, protection of proprietary technology, reliance on third party organizations, risks of obtaining regulatory approval for any product candidate that it may develop, development by competitors of technological innovations, compliance with government regulations, and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s 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 conformity with generally accepted accounting principles in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). The accompanying consolidated financial statements and footnotes to the financial statements have been prepared on the same basis as the most recently audited annual consolidated financial statements and, in the opinion of management, reflect all normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of March 31, 2020 and the results of its operations and its cash flows for the three months ended March 31, 2020 and 2019. The results for the three months ended March 31, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on March 5, 2020 (the “Annual Report on Form 10-K”). Initial Public Offering On July 22, 2019, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 4,500,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $63.9 million after deducting underwriting discounts and commissions and estimated offering expenses. Upon the closing of the IPO, all 112,500,000 shares of outstanding preferred stock automatically converted into 16,071,418 shares of common stock. On July 5, 2019, in connection with the IPO, the Company effected a one-for-seven reverse stock split of the Company’s issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each of the Company’s outstanding series of preferred stock. All share and per share amounts in the accompanying consolidated financial statements and notes thereto for periods prior to the reverse stock split have been retroactively adjusted to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Liquidity The Company has incurred recurring losses and negative cash flows from operations since inception and has primarily funded its operations with proceeds from the IPO, issuances of convertible notes and convertible preferred stock, and an upfront payment received from its collaboration and license agreement (the “Acceleron Collaboration Agreement”) with Acceleron Pharma Inc. (“Acceleron”). As of March 31, 2020, the Company had an accumulated deficit of $169.3 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to expand its research and development efforts. The Company expects to finance its future cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. As of the date of issuance of these financial statements, the Company expects that its cash, cash equivalents, and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of issuance of these financial statements. However, the Company has based this estimate on assumptions that may prove to be wrong, and its operating plan may change as a result of many factors currently unknown to it. As a result, the Company could deplete its capital resources sooner than it currently expects. If the Company is unable to raise additional funds through equity or debt financings when needed, it may be required to delay, limit, reduce or terminate development or future commercialization efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, Fulcrum Therapeutics Securities Corp., which is a Massachusetts subsidiary created to buy, sell, and hold securities. All intercompany transactions and balances have been eliminated. Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the accompanying consolidated financial statements are described in the Company’s audited consolidated financial statements for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K Use of Estimates The preparation of financial statements in accordance with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements, and the reported amount of expenses during the reported periods. Estimates inherent in the preparation of these consolidated financial statements include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock-based compensation expense, the fair value of the common stock and convertible preferred stock prior to the completion of the Company’s IPO, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions. Fair Value of Financial Instruments The fair value of the Company’s financial assets and liabilities reflects the Company’s estimate of amounts that it would have received in connection with the sale of the assets or paid in connection with the transfer of the liabilities in an orderly transaction between market participants at the measurement date. In connection with measuring the fair value of its assets and liabilities, the Company seeks to maximize the use of observable inputs (market data obtained from sources independent from the Company) and to minimize the use of unobservable inputs (the Company’s assumptions about how market participants would price assets and liabilities). The following fair value hierarchy is used to classify assets and liabilities based on the observable inputs and unobservable inputs used in order to value the assets and liabilities: Level 1: Quoted prices in active markets for identical assets or liabilities. An active market for an asset or liability is a market in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis. Level 2: Observable inputs other than Level 1 inputs. Examples of Level 2 inputs include quoted prices in active markets for similar assets or liabilities and quoted prices for identical assets or liabilities in markets that are not active. Level 3: Unobservable inputs based on the Company’s assessment of the assumptions that market participants would use in pricing the asset or liability. The Company’s cash equivalents and marketable securities are carried at fair value and are classified according to the fair value hierarchy described above (see Note 3). The cash equivalents and marketable securities are initially valued at the transaction price, and subsequently revalued at the end of each reporting period, utilizing third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market based approaches, to determine fair value. Marketable Securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as marketable securities. As of March 31, 2020, the Company’s marketable securities consisted of investments in U.S. Treasury securities, corporate bonds, and commercial paper. Marketable securities are classified as current assets on the consolidated balance sheets if the marketable securities are available to be converted into cash to fund current operations. Marketable securities are carried at fair value with the unrealized gains and losses included in accumulated other comprehensive loss, which is a component of stockholders’ equity, until such gains and losses are realized. Any premium arising at purchase is amortized to interest expense over the period of the earliest call date, and any discount arising at purchase is accreted to interest income over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income, net. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss. Off-Balance Sheet Risk and Concentrations of Credit Risk The Company has no significant off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities, and restricted cash. The Company’s cash, cash equivalents, and restricted cash are deposited in accounts at large financial institutions. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash, cash equivalents and restricted cash are held. The Company maintains its cash equivalents in money market funds that invest in U.S. Treasury securities. The Company’s marketable securities primarily consist of U.S. Treasury securities, corporate bonds, and commercial paper, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment. The Company has not experienced any credit losses and does not believe it is exposed to any significant credit risk on these funds. Recently Adopted Accounting Pronouncements In March 2017, the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 Recent Accounting Pronouncements—To Be Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the fair value hierarchy classification of such fair values as of March 31, 2020 and December 31, 2019 (in thousands): Fair Value Measurements at March 31, 2020 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 20,585 $ 20,585 $ — $ — Commercial paper 4,794 — 4,794 — Marketable securities: U.S. Treasury securities 4,042 — 4,042 — Corporate bonds 23,302 — 23,302 — Commercial paper 28,484 — 28,484 — Total $ 81,207 $ 20,585 $ 60,622 $ — Fair Value Measurements at December 31, 2019 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 96,713 $ 96,713 $ — $ — Total $ 96,713 $ 96,713 $ — $ — There were no transfers between fair value levels during the three months ended March 31, 2020.

Marketable Securities

Marketable Securities	3 Months Ended
	Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities Marketable securities consisted of the following as of March 31, 2020 (in thousands): Fair Value Measurements at March 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash equivalents: Money market funds $ 20,585 $ — $ — $ 20,585 Commercial paper 4,794 — — 4,794 Total cash equivalents 25,379 — — 25,379 Marketable securities: U.S. Treasury securities 4,017 25 — 4,042 Corporate bonds 23,380 2 (80 ) 23,302 Commercial paper 28,484 — — 28,484 Total marketable securities 55,881 27 (80 ) 55,828 Total cash equivalents and marketable securities $ 81,260 $ 27 $ (80 ) $ 81,207 The Company did not hold any marketable securities as of December 31, 2019. There were no sales of marketable securities during the three months ended March 31, 2020. As of March 31, 2020, the aggregate fair value of securities that were in an unrealized loss position for less than twelve months was $21.8 million. As of March 31, 2020, no securities were in an unrealized loss position for more than twelve months. The Company determined that it did not hold any securities with any other-than-temporary impairment as of March 31, 2020. As of March 31, 2020, the remaining contractual maturity of all of the Company’s marketable securities is less than one year.

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): March 31, 2020 December 31, 2019 Lab equipment $ 6,327 $ 5,710 Furniture and fixtures 594 548 Computer equipment 373 512 Software 199 90 Leasehold improvements 6,210 6,210 Construction in process — 82 Total property and equipment 13,703 13,152 Less: accumulated depreciation (4,341 ) (3,947 ) Property and equipment, net $ 9,362 $ 9,205 Depreciation expense for each of the three months ended March 31, 2020 and 2019 was $0.5 million.

Additional Balance Sheet Detail

Additional Balance Sheet Detail	3 Months Ended
	Mar. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Additional Balance Sheet Detail	6. Additional Balance Sheet Detail Prepaid expenses and other current assets consisted of the following (in thousands): March 31, 2020 December 31, 2019 Prepaid expenses $ 2,176 $ 2,796 Prepaid sign-on bonuses subject to vesting provisions 147 179 Interest income receivable 177 111 Other 291 284 Total prepaid expenses and other current assets $ 2,791 $ 3,370 Accrued expenses and other current liabilities consisted of the following (in thousands): March 31, 2020 December 31, 2019 External research and development $ 3,988 $ 2,250 Payroll and benefits 781 2,239 Professional services 326 891 Capital lease obligation, current portion 51 50 Restricted stock liability, current portion 14 17 Other 59 49 Total accrued expenses and other current liabilities $ 5,219 $ 5,496

Preferred Stock

Preferred Stock	3 Months Ended
	Mar. 31, 2020
Temporary Equity Disclosure [Abstract]
Preferred Stock	7. Preferred Stock As of March 31, 2020 and December 31, 2019, 5,000,000 shares of preferred stock were authorized. No shares of preferred stock were issued or outstanding as of March 31, 2020 and December 31, 2019. During the three months ended March 31, 2019, the Company issued 12,500,000 shares of Series B convertible preferred stock (the “Series B Preferred Stock”) in connection with the GSK Agreement (Note 11). The rights, privileges, and preferences of the Series B Preferred Stock issued in connection with the GSK Agreement were consistent with the rights, privileges, and preferences of the Series B Preferred Stock issued during prior periods. On July 5, 2019, the Company eliminated the gross proceeds threshold of $45.0 million for a firm-commitment underwritten public offering that would result in the automatic conversion of all outstanding shares Series A convertible preferred stock and Series B Preferred Stock (together with the Series A convertible preferred stock, the “Preferred Stock”). Upon the completion of the IPO on July 22, 2019, all 112,500,000 shares of outstanding Preferred Stock automatically converted into 16,071,418 shares of common stock. In addition, upon the completion of the IPO, the Company amended and restated its certificate of incorporation to authorize 5,000,000 shares of preferred stock, which shares of preferred stock are currently undesignated. No dividends have been declared since inception.

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2020
Equity [Abstract]
Common Stock	8. Common Stock As of March 31, 2020 and December 31, 2019, the Company’s restated certificate of incorporation authorized the Company to issue 200,000,000 shares of common stock, $0.001 par value per share. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the Company’s board of directors, subject to the preferential dividend rights of any preferred stock then outstanding. No dividends have been declared or paid by the Company since its inception. As of March 31, 2020 and December 31, 2019 the Company has reserved for future issuance the following number of shares of common stock: March 31, 2020 December 31, 2019 Shares reserved for exercises of outstanding stock options 2,832,467 2,023,828 Shares reserved for future issuance under the 2019 Stock Incentive Plan 1,963,790 1,866,694 Shares reserved for future issuance under the 2019 Employee Stock Purchase Plan 485,497 252,142 5,281,754 4,142,664

Stock-based Compensation Expens

Stock-based Compensation Expense	3 Months Ended
	Mar. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation Expense	9. Stock-based Compensation Expense 2016 Stock Incentive Plan In July 2016, the Company adopted the 2016 Stock Incentive Plan (the “2016 Plan”), which provided for the grant of restricted stock awards, restricted stock units, incentive stock options, non-statutory stock options, and other stock-based awards to the Company’s eligible employees, officers, directors, consultants, and advisors. As of the effective date of the 2019 Stock Incentive Plan (the “2019 Plan”), and as of March 31, 2020, no shares remained available for future issuance under the 2016 Plan. Any options or awards outstanding under the 2016 Plan remain outstanding and effective. 2019 Stock Incentive Plan On July 2, 2019, the Company’s stockholders approved the 2019 Plan, which became effective on July 17, 2019. The 2019 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to the Company’s officers, employees, directors, consultants and advisors. The number of shares initially reserved for issuance under the 2019 Plan is 2,017,142 shares, plus the shares of common stock remaining available for issuance under the 2016 Plan as of July 17, 2019. The number of shares reserved shall be annually increased on January 1, 2020 and each January 1 thereafter through January 1, 2029 by the least of (i) 2,000,000 shares, (ii) 4% of the number of shares of the Company’s common stock outstanding on the first day of each such year or (iii) an amount determined by the Company’s board of directors. On January 1, 2020, the number of shares reserved for issuance under the 2019 Plan was increased by 933,420 shares. As of March 31, 2020, there were 1,963,790 shares available for future issuance under the 2019 Plan. The shares of common stock underlying any awards that expire, terminate, or are otherwise surrendered, cancelled, forfeited or repurchased by the Company under the 2016 Plan or the 2019 Plan will be added back to the shares of common stock available for issuance under the 2019 Plan. As of July 17, 2019, no further awards will be made under the 2016 Plan. For financial reporting purposes, the Company performed common stock valuations with the assistance of a third-party specialist as of May 10, 2019, March 15, 2019, November 30, 2018, August 24, 2018, June 1, 2018, December 31, 2017, and December 31, 2016 to determine stock-based compensation expense for restricted stock awards and stock options. Upon completion of the IPO, the fair value of the common stock on the grant date was based on the closing price of the stock on the Nasdaq Global Market on the date of grant. The Company may repurchase unvested shares at the original purchase price if employees or non-employees are terminated or cease their employment or service relationship with the Company. Shares of common stock repurchased from employees and non-employees are shares held in the Company’s treasury (“Treasury Shares”). The board of directors may, at its discretion, authorize that the Treasury Shares be returned to the pool of authorized but unissued common stock. The shares of common stock underlying restricted stock awards typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest. The following table summarizes the Company’s restricted stock activity under the 2019 Plan and 2016 Plan since December 31, 2019: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2019 346,423 $ 3.05 Granted — — Vested (77,758 ) 3.02 Repurchased (8,787 ) 3.02 Unvested at March 31, 2020 259,878 $ 3.06 Stock options granted by the Company typically vest over a four-year period and have a ten year contractual term. The following table summarizes the Company’s stock option activity under the 2019 Plan and 2016 Plan since December 31, 2019: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2019 2,023,828 $ 9.31 8.94 $ 2,730,209 Granted 853,281 16.68 Exercised (36,472 ) 7.84 Cancelled (8,170 ) 11.45 Outstanding at March 31, 2020 2,832,467 $ 11.54 9.17 $ 6,431,439 Exercisable at March 31, 2020 418,971 $ 8.14 8.73 $ 1,730,138 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 as of the balance sheet date for those options that had exercise prices lower than the fair value of the Company’s common stock. The weighted average grant date fair value of stock options granted during the three months ended March 31, 2020 was $11.06 per share. The weighted average grant date fair value of stock options granted during the three months ended March 31, 2019 was $5.56 per share. The total intrinsic value of stock options exercised during the three months ended March 31, 2020 was $0.3 million. No stock options were exercised during the three months ended March 31, 2019. The fair value of stock options granted during the three months ended March 31, 2020 and 2019 under the 2019 Plan and the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions: Three Months Ended March 31, 2020 Three Months Ended March 31, 2019 Risk-free interest rate 1.6 % 2.6 % Expected dividend yield 0.0 % 0.0 % Expected term (years) 6.0 6.0 Expected stock price volatility 75.6 % 81.4 % Grants Outside of Stock Incentive Plans The following table summarizes the Company’s restricted stock activity outside of the 2019 Plan and 2016 Plan since December 31, 2019: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2019 334,647 $ 2.94 Granted — — Vested (24,463 ) 2.94 Repurchased — — Unvested at March 31, 2020 310,184 $ 2.94 The aggregate intrinsic value of all restricted stock awards that vested during the three months ended March 31, 2020 and 2019 was $1.6 million and $1.4 million, respectively. Stock-based Compensation Expense The total compensation cost recognized in the statements of operations associated with all stock-based compensation awards granted by the Company is as follows (in thousands): Three Months Ended March 31, 2020 2019 Research and development $ 758 $ 406 General and administrative 935 415 Total stock-based compensation expense $ 1,693 $ 821 As of March 31, 2020, the Company had an aggregate of $19.8 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted average period of 2.97 years. 2019 Employee Stock Purchase Plan On July 2, 2019, the Company’s stockholders approved the 2019 Employee Stock Purchase Plan (the “ESPP”), which became effective on July 17, 2019. A total of 252,142 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock reserved under the ESPP shall be annually increased on January 1, 2020, and each January 1 thereafter through January 1, 2029, by the least of (i) 428,571 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the first day of each such year or (iii) an amount determined by the Company’s board of directors. On January 1, 2020, the number of shares reserved for issuance under the 2019 ESPP was increased by 233,355 shares.

Acceleron Collaboration Agreeme

Acceleron Collaboration Agreement	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Acceleron Collaboration Agreement	10. Acceleron Collaboration Agreement On December 20, 2019, the Company entered into the Acceleron Collaboration Agreement to identify biological targets to modulate specific pathways associated with a targeted indication within the pulmonary disease space (the “Indication”). Under the terms of the Acceleron Collaboration Agreement, the Company granted Acceleron an exclusive worldwide license under certain intellectual property rights to make, have made, use, sell, have sold, import, export, distribute and have distributed, market, have marketed, promote, have promoted, or otherwise exploit molecules and products directed against or expressing certain biological targets identified by the Company for the treatment, prophylaxis, or diagnosis of the Indication. Pursuant to a mutually agreed research plan, the Company will perform assay screening and related research activities to identify and validate potential biological targets for further research in order to support the development, manufacture and commercialization of product candidates by Acceleron. Upon completion of the research activities, the Company will deliver a data package to Acceleron with respect to the biological targets identified by the Company in the conduct of the research activities for the treatment, prophylaxis, or diagnosis of the Indication. As provided for under the exclusive worldwide license that was conveyed at the inception of the arrangement, Acceleron has the right to designate a specified number of the biological targets identified by the Company for Acceleron’s research, development, manufacture and commercialization of products or molecules directed to such targets for the treatment, prophylaxis, or diagnosis of the Indication (the “Targets”). If Acceleron does not designate any Targets during the designated period, then the Agreement will automatically terminate. If Acceleron designates one or more Targets, then Acceleron will be obligated to use commercially reasonable efforts to seek regulatory approval for one product directed to a Target in certain specified countries. Upon receipt of regulatory approval for any product directed to a Target, Acceleron must use commercially reasonable efforts to commercialize such product in certain specified countries. Acceleron may also request that the Company perform medicinal chemistry services related to the generation and optimization of molecules directed against or expressing biological targets for the treatment, prophylaxis, or diagnosis of the Indication beyond the scope of the research plan. If the Company agrees to provide such medicinal chemistry services, the Company and Acceleron will negotiate to determine the scope, timeline and budget for such medicinal chemistry services. The Company received a non-refundable upfront payment of $10.0 million in December 2019 upon the execution of the Acceleron Collaboration Agreement. The Company will be entitled to research milestone payments of up to $18.5 million in the aggregate upon achievement of specified research milestones, development milestone payments of up to $202.5 million in the aggregate upon achievement of specified clinical and regulatory milestones, and sales milestones payments of up to $217.5 million in the aggregate upon the achievement of certain aggregate annual worldwide net sales milestones for certain products directed to a Target that have achieved such milestones. In addition, the Company will be entitled to tiered royalties ranging from a mid single-digit percentage to a low double-digit percentage on Acceleron’s annual worldwide net sales of products directed to any Target, subject to reduction in specified circumstances. The Company is also entitled to receive reimbursement from Acceleron for research costs incurred under the research plan, including internal and external costs. The Acceleron Collaboration Agreement continues on a country-by-country and Target-by-Target basis until the last to expire royalty term for a product directed to such Target, at which time the Acceleron Collaboration Agreement expires with respect to such Target in such country. Either party has the right to terminate the Acceleron Collaboration Agreement if the other party has materially breached in the performance of its obligations under the contract and such breach has not been cured within the applicable cure period. Acceleron also has the right to terminate the Acceleron Collaboration Agreement for convenience in its entirety or on a Target-by-Target and, if the Company performs medicinal chemistry services, on a molecule-by-molecule basis with respect to any molecule directed against a Target. While the Company is performing the research activities pursuant to the research plan and for a specified period thereafter, the Company may not research, develop, manufacture, commercialize, use, or otherwise exploit any compound or product for the treatment, prophylaxis, or diagnosis of the Indication other than for Acceleron. While the Company is performing the research activities pursuant to the research plan and for a specified period thereafter, other than for Acceleron, the Company may not research, develop, manufacture, commercialize, use, or otherwise exploit any compound or product for the treatment, prophylaxis, or diagnosis of the Indication that is directed against certain specified biological targets identified by the Company in the performance of the research activities. Accounting Analysis Identification of the Contract The Company assessed the Acceleron Collaboration Agreement and concluded that it represents a contract with a customer within the scope of ASC 606. Identification of the Promises and Performance Obligations The Company determined that the Acceleron Collaboration Agreement contains the following promises: (i) an exclusive worldwide license under certain intellectual property rights, including rights to a specified number of biological targets identified by the Company for the treatment, prophylaxis, or diagnosis of a targeted indication within the pulmonary disease space that was conveyed at the inception of the arrangement (the “License”), (ii) research services to identify and validate potential biological targets (the “Research Services”), and (iii) participation in the joint steering committee (the “JSC”). The Company assessed the above promises and concluded that the License is not capable of being distinct from the Research Services given that the License has limited value without the performance of the Research Services and the Research Services can only be performed by the Company due to their specialized nature. Therefore, the Company has concluded that the License and the Research Services represent a single combined performance obligation. The Company also assessed the participation on the JSC and concluded that the promise is quantitatively and qualitatively immaterial in the context of the Acceleron Collaboration Agreement. Accordingly, the Company has disregarded its participation on the JSC as a performance obligation. The potential medicinal chemistry services were not identified as a promised good or service because the Company is under no obligation to provide those services. Determination of the Transaction Price The Company received a non-refundable upfront payment of $10.0 million upon the execution of the Acceleron Collaboration Agreement, which the Company included in the transaction price. Based on the uncertainty associated with the achievement of any research and development milestone payments that the Company is eligible to receive, the Company has constrained the variable consideration associated with those milestone payments and excluded them from the transaction price. As part of its evaluation of constraining the research and development milestones, the Company considered numerous factors, including the fact that the achievement of the research and development milestones are contingent upon the results of the underlying research and development activities and are thus outside of the control of the Company. The Company also included in the transaction price the expected amount of costs to be reimbursed for the Research Services. The Company reassesses the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjusts its estimate of the transaction price. There was no change in the amount of variable consideration constrained during the three months ended March 31, 2020. Any consideration related to sales milestone payments (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to Acceleron and therefore are recognized at the later of when the related sales occur or the performance obligation is satisfied. Allocation of the Transaction Price to Performance Obligations As noted above, the Company has identified a single performance obligation associated with the Acceleron Collaboration Agreement. Therefore, the Company will allocate the entire amount of the transaction price to the identified single performance obligation. Recognition of Revenue The Company recognizes revenue related to the Acceleron Collaboration Agreement over time as the Research Services are rendered. The Company has concluded that an input method is a representative depiction of the transfer of services under the Acceleron Collaboration Agreement. The method of measuring progress towards the delivery of the services incorporates actual cumulative internal and external costs incurred relative to total internal and external costs expected to be incurred to satisfy the performance obligation. The period over which total costs are estimated reflects the Company’s estimate of the period over which it will perform the Research Services. Changes in estimates of total internal and external costs expected to be incurred are recognized in the period of change as a cumulative catch-up adjustment. During the three months ended March 31, 2020, the Company recognized $0.8 million of collaboration revenue associated with the Acceleron Collaboration Agreement. As of December 31, 2019, no Research Services had been performed. For the three months ended March 31, 2019, the Company did not recognize any revenue under the Acceleron Collaboration Agreement as it had not yet entered into the Acceleron Collaboration Agreement. As of March 31, 2020 and December 31, 2019, the Company recorded deferred revenue of $9.6 million and $10.0 million, respectively, which is classified as either current or net of current portion in the accompanying consolidated balance sheets based on the period over which the revenue is expected to be recognized. The aggregate deferred revenue balance represents the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of March 31, 2020 and December 31, 2019, respectively. As of March 31, 2020 and December 31, 2019, the Company had not received any milestone, royalty, or cost reimbursement payments under the Acceleron Collaboration Agreement. As of March 31, 2020, the Company recorded unbilled accounts receivable of $0.3 million related to reimbursable Research Services costs under the Acceleron Collaboration Agreement for activities performed during the three months ended March 31, 2020.

Asset Acquisition

Asset Acquisition	3 Months Ended
	Mar. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Asset Acquisition	11. Asset Acquisition In February 2019, the Company entered into a right of reference and license agreement (the “GSK Agreement”) with subsidiaries of GlaxoSmithKline plc (collectively referred to as “GSK”), pursuant to which the Company has been granted an exclusive worldwide license to develop and commercialize losmapimod. Under the GSK Agreement, the Company also acquired reference rights to relevant regulatory and manufacturing documents and GSK’s existing supply of losmapimod drug substance and product. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The Company is obligated to use commercially reasonable efforts to develop and commercialize losmapimod at its sole cost. The Company is also responsible for costs related to the filing and maintenance of the licensed patent rights. Under the GSK Agreement, the Company issued 12,500,000 million shares of Series B Preferred Stock to GSK with an estimated fair value of $25.5 million, or $2.04 per share, which was determined with the assistance of a third-party specialist contemporaneously with the issuance of the Series B Preferred Stock to GSK. In addition, the Company may owe GSK up to $37.5 million in certain specified clinical and regulatory milestones, including $2.5 million due upon the initiation of a Phase 2 clinical trial, which was achieved and paid during the third quarter of 2019, and up to $60.0 million in certain specified sales milestones. The Company has agreed to pay tiered royalties on annual net sales of losmapimod that range from mid single-digit percentages to a low double-digit, but less than teens, percentage. The royalties are payable on a product-by-product and country-by-country basis, and may be reduced in specified circumstances. The Company also incurred $0.1 million of direct expenses related to the transaction, which the Company included in the total consideration for the transaction. The GSK Agreement may be terminated by either party for a material breach by the other, subject to notice and cure provisions. Unless earlier terminated, the GSK Agreement will continue in effect until the expiration of the Company’s royalty obligations, which expire on a country-by-country basis on the later of (i) ten years after the first commercial sale in the country or (ii) approval of a generic version of losmapimod by the applicable regulatory agency. The Company concluded the arrangement did not result in the acquisition of a business, as substantially all of the fair value of the gross assets acquired was concentrated in a single in-process research and development asset, losmapimod. In addition, the Company did not obtain any substantive processes in connection to the GSK Agreement and losmapimod was not generating revenue at the time the GSK Agreement was executed. Therefore, the Company accounted for the arrangement as an asset acquisition. The Company also concluded that the acquired assets do not have an alternative future use, and therefore the fair value attributable to the GSK Agreement of $25.6 million, inclusive of transaction costs, was recorded as in-process research and development expense (a component of research and development expenses) in the Company’s consolidated statement of operations and comprehensive loss for the three months ended March 31, 2019, which is the period in which the Company obtained (i) the license to losmapimod, (ii) the right to reference relevant regulatory and manufacturing documents, and (iii) GSK’s existing supply of losmapimod drug substance and product. Additionally, the Company will recognize clinical and regulatory milestone payments when the underlying contingency is resolved and the consideration is paid or becomes payable. The milestone payments will be capitalized or expensed depending on the nature of the associated asset as of the date of recognition. The Company will record sales milestone payments and royalties as additional expense of the related product sales in the period in which the corresponding sales occur.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Operating Leases In November 2017, the Company entered into a lease agreement for its current corporate headquarters for approximately 28,731 square feet of office and laboratory space in Cambridge, Massachusetts. The lease has a total commitment of $25.1 million over the ten year term, and includes escalating rent payments. The lease agreement requires the Company to either pay a security deposit or maintain a letter of credit of $1.1 million. The Company maintains a letter of credit for this lease and has recorded the cash held to secure the letter of credit as restricted cash on the consolidated balance sheet as of March 31, 2020 and December 31, 2019. Rent expense associated with this lease for each of the three months ended March 31, 2020 and 2019 was approximately $0.5 million. The future minimum lease payments associated with the lease for the Company’s current headquarters as of March 31, 2020, are as follows (in thousands): 2020 (1) $ 1,723 2021 2,354 2022 2,424 2023 2,497 2024 2,572 Thereafter 9,615 Total minimum lease payments $ 21,185 (1) Amounts are for the nine months ending December 31, 2020. Other Agreements The Company has agreements with third parties in the normal course of business under which it can license certain developed technologies. If the Company exercises its rights to license the technologies it may be subject to additional fees and milestone payments. As of March 31, 2020, the Company has not exercised its rights to license such technologies. 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 arising out of the relationship between such parties and the Company. In addition, the Company has entered into indemnification agreements with members of its board of directors and senior management 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 aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations as of March 31, 2020 or December 31, 2019. Legal Proceedings The Company is not currently a party to any material legal proceedings. 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 expenses the costs related to its legal proceedings as they are incurred. No such costs have been incurred during the three months ended March 31, 2020 and 2019.

Defined Contribution Plan

Defined Contribution Plan	3 Months Ended
	Mar. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	13. Defined Contribution Plan The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants the option to elect to defer a portion of their annual compensation on a pretax basis. As currently established, the Company is not required to make and has not made any contributions to the 401(k) Plan.

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Net Loss per Share	14. Net Loss per Share The following common stock equivalents were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended March 31, 2020 2019 Series A Preferred Stock — 8,571,427 Series B Preferred Stock — 7,499,991 Outstanding stock options 2,832,467 1,528,387 Unvested restricted stock awards 570,062 1,025,876 Total 3,402,529 18,625,681

Related-Party Transactions

Related-Party Transactions	3 Months Ended
	Mar. 31, 2020
Related Party Transactions [Abstract]
Related-Party Transactions	15. Related-Party Transactions During the three months ended March 31, 2019, the Company paid fees to Third Rock Ventures, LLC (“TRV”), an affiliate of one of the Company’s principal stockholders, in exchange for consulting services. During the three months ended March 31, 2020, the Company did not pay fees to TRV in exchange for services. The Company recorded expenses related to such fees of less than $0.1 million during the three months ended March 31, 2019. The Company did not record expenses related to such fees during the three months ended March 31, 2020. As of December 31, 2019, there was less than $0.1 million of amounts due to TRV for such services that were included in accounts payable and accrued expenses. As of March 31, 2020, there were no amounts due to TRV for such services that were included in accounts payable and accrued expenses. Additionally, consultants that provide services to the Company are employees of TRV. The Company has issued an aggregate of 142,284 shares of common stock to these consultants in exchange for their continuing consulting services.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, Fulcrum Therapeutics Securities Corp., which is a Massachusetts subsidiary created to buy, sell, and hold securities. All intercompany transactions and balances have been eliminated.
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the accompanying consolidated financial statements are described in the Company’s audited consolidated financial statements for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K
Use of Estimates	Use of Estimates The preparation of financial statements in accordance with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements, and the reported amount of expenses during the reported periods. Estimates inherent in the preparation of these consolidated financial statements include, but are not limited to, estimates related to revenue recognition, accrued expenses, stock-based compensation expense, the fair value of the common stock and convertible preferred stock prior to the completion of the Company’s IPO, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The fair value of the Company’s financial assets and liabilities reflects the Company’s estimate of amounts that it would have received in connection with the sale of the assets or paid in connection with the transfer of the liabilities in an orderly transaction between market participants at the measurement date. In connection with measuring the fair value of its assets and liabilities, the Company seeks to maximize the use of observable inputs (market data obtained from sources independent from the Company) and to minimize the use of unobservable inputs (the Company’s assumptions about how market participants would price assets and liabilities). The following fair value hierarchy is used to classify assets and liabilities based on the observable inputs and unobservable inputs used in order to value the assets and liabilities: Level 1: Quoted prices in active markets for identical assets or liabilities. An active market for an asset or liability is a market in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis. Level 2: Observable inputs other than Level 1 inputs. Examples of Level 2 inputs include quoted prices in active markets for similar assets or liabilities and quoted prices for identical assets or liabilities in markets that are not active. Level 3: Unobservable inputs based on the Company’s assessment of the assumptions that market participants would use in pricing the asset or liability. The Company’s cash equivalents and marketable securities are carried at fair value and are classified according to the fair value hierarchy described above (see Note 3). The cash equivalents and marketable securities are initially valued at the transaction price, and subsequently revalued at the end of each reporting period, utilizing third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market based approaches, to determine fair value.
Marketable Securities	Marketable Securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as marketable securities. As of March 31, 2020, the Company’s marketable securities consisted of investments in U.S. Treasury securities, corporate bonds, and commercial paper. Marketable securities are classified as current assets on the consolidated balance sheets if the marketable securities are available to be converted into cash to fund current operations. Marketable securities are carried at fair value with the unrealized gains and losses included in accumulated other comprehensive loss, which is a component of stockholders’ equity, until such gains and losses are realized. Any premium arising at purchase is amortized to interest expense over the period of the earliest call date, and any discount arising at purchase is accreted to interest income over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income, net. If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s statement of operations and comprehensive loss.
Off-Balance Sheet Risk and Concentrations of Credit Risk	Off-Balance Sheet Risk and Concentrations of Credit Risk The Company has no significant off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities, and restricted cash. The Company’s cash, cash equivalents, and restricted cash are deposited in accounts at large financial institutions. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash, cash equivalents and restricted cash are held. The Company maintains its cash equivalents in money market funds that invest in U.S. Treasury securities. The Company’s marketable securities primarily consist of U.S. Treasury securities, corporate bonds, and commercial paper, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment. The Company has not experienced any credit losses and does not believe it is exposed to any significant credit risk on these funds.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In March 2017, the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606
Recent Accounting Pronouncements-To Be Adopted	Recent Accounting Pronouncements—To Be Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Classification	The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the fair value hierarchy classification of such fair values as of March 31, 2020 and December 31, 2019 (in thousands): Fair Value Measurements at March 31, 2020 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 20,585 $ 20,585 $ — $ — Commercial paper 4,794 — 4,794 — Marketable securities: U.S. Treasury securities 4,042 — 4,042 — Corporate bonds 23,302 — 23,302 — Commercial paper 28,484 — 28,484 — Total $ 81,207 $ 20,585 $ 60,622 $ — Fair Value Measurements at December 31, 2019 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 96,713 $ 96,713 $ — $ — Total $ 96,713 $ 96,713 $ — $ —

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]
Summary of Marketable Securities	Marketable securities consisted of the following as of March 31, 2020 (in thousands): Fair Value Measurements at March 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash equivalents: Money market funds $ 20,585 $ — $ — $ 20,585 Commercial paper 4,794 — — 4,794 Total cash equivalents 25,379 — — 25,379 Marketable securities: U.S. Treasury securities 4,017 25 — 4,042 Corporate bonds 23,380 2 (80 ) 23,302 Commercial paper 28,484 — — 28,484 Total marketable securities 55,881 27 (80 ) 55,828 Total cash equivalents and marketable securities $ 81,260 $ 27 $ (80 ) $ 81,207

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): March 31, 2020 December 31, 2019 Lab equipment $ 6,327 $ 5,710 Furniture and fixtures 594 548 Computer equipment 373 512 Software 199 90 Leasehold improvements 6,210 6,210 Construction in process — 82 Total property and equipment 13,703 13,152 Less: accumulated depreciation (4,341 ) (3,947 ) Property and equipment, net $ 9,362 $ 9,205

Additional Balance Sheet Deta_2

Additional Balance Sheet Detail (Tables)	3 Months Ended
	Mar. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): March 31, 2020 December 31, 2019 Prepaid expenses $ 2,176 $ 2,796 Prepaid sign-on bonuses subject to vesting provisions 147 179 Interest income receivable 177 111 Other 291 284 Total prepaid expenses and other current assets $ 2,791 $ 3,370
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): March 31, 2020 December 31, 2019 External research and development $ 3,988 $ 2,250 Payroll and benefits 781 2,239 Professional services 326 891 Capital lease obligation, current portion 51 50 Restricted stock liability, current portion 14 17 Other 59 49 Total accrued expenses and other current liabilities $ 5,219 $ 5,496

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended
	Mar. 31, 2020
Equity [Abstract]
Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock	As of March 31, 2020 and December 31, 2019 the Company has reserved for future issuance the following number of shares of common stock: March 31, 2020 December 31, 2019 Shares reserved for exercises of outstanding stock options 2,832,467 2,023,828 Shares reserved for future issuance under the 2019 Stock Incentive Plan 1,963,790 1,866,694 Shares reserved for future issuance under the 2019 Employee Stock Purchase Plan 485,497 252,142 5,281,754 4,142,664

Stock-based Compensation Expe_2

Stock-based Compensation Expense (Tables)	3 Months Ended
	Mar. 31, 2020
Summary of Stock Option Activity	Stock options granted by the Company typically vest over a four-year period and have a ten year contractual term. The following table summarizes the Company’s stock option activity under the 2019 Plan and 2016 Plan since December 31, 2019: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2019 2,023,828 $ 9.31 8.94 $ 2,730,209 Granted 853,281 16.68 Exercised (36,472 ) 7.84 Cancelled (8,170 ) 11.45 Outstanding at March 31, 2020 2,832,467 $ 11.54 9.17 $ 6,431,439 Exercisable at March 31, 2020 418,971 $ 8.14 8.73 $ 1,730,138
Weighted Average Assumptions Used to Calculate Fair Value of Stock Option	The fair value of stock options granted during the three months ended March 31, 2020 and 2019 under the 2019 Plan and the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions: Three Months Ended March 31, 2020 Three Months Ended March 31, 2019 Risk-free interest rate 1.6 % 2.6 % Expected dividend yield 0.0 % 0.0 % Expected term (years) 6.0 6.0 Expected stock price volatility 75.6 % 81.4 %
Summary of Stock-Based Compensation Expense Recognized	The total compensation cost recognized in the statements of operations associated with all stock-based compensation awards granted by the Company is as follows (in thousands): Three Months Ended March 31, 2020 2019 Research and development $ 758 $ 406 General and administrative 935 415 Total stock-based compensation expense $ 1,693 $ 821
2019 Plan and 2016 Plan
Summary of Restricted Stock Activity	The shares of common stock underlying restricted stock awards typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest. The following table summarizes the Company’s restricted stock activity under the 2019 Plan and 2016 Plan since December 31, 2019: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2019 346,423 $ 3.05 Granted — — Vested (77,758 ) 3.02 Repurchased (8,787 ) 3.02 Unvested at March 31, 2020 259,878 $ 3.06
Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan
Summary of Restricted Stock Activity	The following table summarizes the Company’s restricted stock activity outside of the 2019 Plan and 2016 Plan since December 31, 2019: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2019 334,647 $ 2.94 Granted — — Vested (24,463 ) 2.94 Repurchased — — Unvested at March 31, 2020 310,184 $ 2.94

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	The future minimum lease payments associated with the lease for the Company’s current headquarters as of March 31, 2020, are as follows (in thousands): 2020 (1) $ 1,723 2021 2,354 2022 2,424 2023 2,497 2024 2,572 Thereafter 9,615 Total minimum lease payments $ 21,185 (1) Amounts are for the nine months ending December 31, 2020.

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended March 31, 2020 2019 Series A Preferred Stock — 8,571,427 Series B Preferred Stock — 7,499,991 Outstanding stock options 2,832,467 1,528,387 Unvested restricted stock awards 570,062 1,025,876 Total 3,402,529 18,625,681

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands	Jul. 22, 2019USD ($)$ / sharesshares	Jul. 05, 2019	Mar. 31, 2020USD ($)shares	Mar. 31, 2019shares	Dec. 31, 2019USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Accumulated deficit | $			$ 169,253		$ 150,801
Convertible Preferred Stock
Nature Of Business And Basis Of Presentation [Line Items]
Convertible preferred stock, share outstanding	112,500,000
Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock, Shares			138,693	134,013
IPO
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock, Shares	4,500,000
Share price | $ / shares	$ 16
Net proceeds of common stock | $	$ 63,900
Reverse stock split		one-for-seven
Stock split		0.14285
IPO | Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Number of preferred stock converted into common stock	16,071,418

Summary of Fair Value Measureme

Summary of Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Classification (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 25,379
Marketable securities	55,828
Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		$ 96,713
Total	81,207
Recurring | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	20,585	96,713
Recurring | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,794
Marketable securities	28,484
Recurring | US Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	4,042
Recurring | Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	23,302
Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		96,713
Total	20,585
Recurring | Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	20,585	$ 96,713
Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	60,622
Recurring | Level 2 | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,794
Marketable securities	28,484
Recurring | Level 2 | US Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	4,042
Recurring | Level 2 | Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 23,302

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details)	Mar. 31, 2020USD ($)
Fair Value Disclosures [Abstract]
Fair value transfers between levels	$ 0

Marketable Securities - Summary

Marketable Securities - Summary of Marketable Securities (Details) $ in Thousands	Mar. 31, 2020USD ($)
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Fair Value	$ 25,379
Marketable securities, Amortized Cost	55,881
Marketable securities, Gross Unrealized Gains	27
Marketable securities, Gross Unrealized Losses	(80)
Marketable securities, Fair Value	55,828
Total cash equivalents and marketable securities, Amortized Cost	81,260
Total cash equivalents and marketable securities, Fair Value	81,207
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Fair Value	20,585
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Fair Value	4,794
Marketable securities, Amortized Cost	28,484
Marketable securities, Fair Value	28,484
US Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	4,017
Marketable securities, Gross Unrealized Gains	25
Marketable securities, Fair Value	4,042
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	23,380
Marketable securities, Gross Unrealized Gains	2
Marketable securities, Gross Unrealized Losses	(80)
Marketable securities, Fair Value	$ 23,302

Marketable Securities - Additio

Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Marketable securities		$ 0
Sale of marketable securities	$ 0
Aggregate fair value of securities, unrealized loss position for less than 12 months	21,800,000
Aggregate fair value of securities, unrealized loss position for more than 12 months	0
Securities with other-than-temporary impairment	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment	$ 13,703	$ 13,152
Less: accumulated depreciation	(4,341)	(3,947)
Property and equipment, net	9,362	9,205
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	6,327	5,710
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	594	548
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	373	512
Software
Property Plant And Equipment [Line Items]
Total property and equipment	199	90
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 6,210	6,210
Construction in Process
Property Plant And Equipment [Line Items]
Total property and equipment		$ 82

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 539	$ 504

Additional Balance Sheet Deta_3

Additional Balance Sheet Detail - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Prepaid expenses	$ 2,176	$ 2,796
Prepaid sign-on bonuses subject to vesting provisions	147	179
Interest income receivable	177	111
Other	291	284
Total prepaid expenses and other current assets	$ 2,791	$ 3,370

Additional Balance Sheet Deta_4

Additional Balance Sheet Detail - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
External research and development	$ 3,988	$ 2,250
Payroll and benefits	781	2,239
Professional services	326	891
Capital lease obligation, current portion	51	50
Restricted stock liability, current portion	14	17
Other	59	49
Total accrued expenses and other current liabilities	$ 5,219	$ 5,496

Preferred Stock - Additional In

Preferred Stock - Additional Information (Details) - USD ($)	Jul. 05, 2019	Mar. 31, 2020	Dec. 31, 2019	Jul. 22, 2019
Temporary Equity [Line Items]
Preferred stock, shares authorized		5,000,000	5,000,000	5,000,000
Preferred stock, shares issued		0	0
Preferred stock, shares outstanding		0	0
Gross proceeds threshold of automatic conversion of preferred stock eliminated	$ 45,000,000
IPO | Common Stock
Temporary Equity [Line Items]
Number of preferred stock converted into common stock				16,071,418
Series B Preferred Stock
Temporary Equity [Line Items]
Temporary equity, dividends declared		$ 0
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, share outstanding				112,500,000
Series A Preferred Stock
Temporary Equity [Line Items]
Temporary equity, dividends declared		$ 0
GSK Agreement | Series B Preferred Stock
Temporary Equity [Line Items]
Issuance of convertible preferred stock		12,500,000

Common Stock - Additional Infor

Common Stock - Additional Information (Details)	3 Months Ended	55 Months Ended
	Mar. 31, 2020Vote$ / sharesshares	Mar. 31, 2020USD ($)Vote$ / sharesshares	Dec. 31, 2019$ / sharesshares
Equity [Abstract]
Common stock, shares authorized | shares	200,000,000	200,000,000	200,000,000
Common stock, par value | $ / shares	$ 0.001	$ 0.001	$ 0.001
Common stock voting rights	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders
Number of common stock voting rights | Vote	1	1
Dividends declared or paid | $		$ 0

Common Stock - Schedule of Pote

Common Stock - Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock (Details) - shares	Mar. 31, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Shares reserved for conversion of preferred stock and future issuance of common stock	5,281,754	4,142,664
Outstanding Stock Options
Class Of Stock [Line Items]
Shares reserved for future issuance	2,832,467	2,023,828
2019 Stock Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	1,963,790	1,866,694
2019 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	485,497	252,142

Stock-based Compensation Expe_3

Stock-based Compensation Expense - Additional Information (Details) - USD ($)	Jan. 01, 2020	Jul. 02, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jul. 17, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense			$ 19,800,000
Unrecognized stock-based compensation expense, weighted average period expect to recognized			2 years 11 months 19 days
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance		252,142
Increase in number of shares reserved for issuance (shares)		428,571
Increase in number of shares reserved for issuance, percent of common stock outstanding		1.00%
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance			2,832,467		2,023,828
2016 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance			0			0
2019 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance		2,017,142	1,963,790
Increase in number of shares reserved for issuance (shares)	933,420	2,000,000
Increase in number of shares reserved for issuance, percent of common stock outstanding		4.00%
2019 Stock Incentive Plan | Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			4 years
Contractual term			10 years
2019 Plan and 2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value			$ 11.06	$ 5.56
Total intrinsic value of stock options exercised				$ 0
2019 Plan and 2016 Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total intrinsic value of stock options exercised			$ 300,000
2019 Plan and 2016 Plan | Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Contractual term			9 years 2 months 1 day		8 years 11 months 8 days
Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of restricted stock awards vested			$ 1,600,000	$ 1,400,000

Stock-based Compensation Expe_4

Stock-based Compensation Expense - Summary of Restricted Stock Activity under 2019 Plan and 2016 Plan (Details) - 2019 Plan and 2016 Plan - Restricted Stock	3 Months Ended
	Mar. 31, 2020$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance | shares	346,423
Number of Shares, Vested | shares	(77,758)
Number of Shares, Repurchased | shares	(8,787)
Number of Shares, Unvested, Ending balance | shares	259,878
Weighted Average Grant Date Fair Value, Unvested, Beginning balance | $ / shares	$ 3.05
Weighted Average Grant Date Fair Value, Vested | $ / shares	3.02
Weighted Average Grant Date Fair Value, Repurchased | $ / shares	3.02
Weighted Average Grant Date Fair Value, Unvested, Ending balance | $ / shares	$ 3.06

Stock-based Compensation Expe_5

Stock-based Compensation Expense - Summary of Stock Option Activity (Details) - 2019 Plan and 2016 Plan - Stock Option - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Outstanding, Beginning balance	2,023,828
Number of Shares, Granted	853,281
Number of Shares, Exercised	(36,472)
Number of Shares, Cancelled	(8,170)
Number of Shares, Outstanding, Ending balance	2,832,467	2,023,828
Number of Shares, Exercisable	418,971
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 9.31
Weighted Average Exercise Price, Granted	16.68
Weighted Average Exercise Price, Exercised	7.84
Weighted Average Exercise Price, Cancelled	11.45
Weighted Average Exercise Price, Outstanding, Ending balance	11.54	$ 9.31
Weighted Average Exercise Price, Exercisable	$ 8.14
Weighted Average Remaining Contractual Term (in years), Outstanding	9 years 2 months 1 day	8 years 11 months 8 days
Weighted Average Remaining Contractual Term (in years), Exercisable	8 years 8 months 23 days
Aggregate Intrinsic Value, Outstanding	$ 6,431,439	$ 2,730,209
Aggregate Intrinsic Value, Exercisable	$ 1,730,138

Stock-based Compensation Expe_6

Stock-based Compensation Expense - Summary of Weighted Average Assumptions Used to Calculate Fair Value of Stock Option (Details) - 2019 Plan and 2016 Plan	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	1.60%	2.60%
Expected dividend yield	0.00%	0.00%
Expected term (years)	6 years	6 years
Expected stock price volatility	75.60%	81.40%

Stock-based Compensation Expe_7

Stock-based Compensation Expense - Summary of Restricted Stock Activity outside of 2019 and 2016 Plan (Details) - Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan - Restricted Stock	3 Months Ended
	Mar. 31, 2020$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance | shares	334,647
Number of Shares, Vested | shares	(24,463)
Number of Shares, Unvested, Ending balance | shares	310,184
Weighted Average Grant Date Fair Value, Unvested, Beginning balance	$ 2.94
Weighted Average Grant Date Fair Value, Granted	0
Weighted Average Grant Date Fair Value, Vested	2.94
Weighted Average Grant Date Fair Value, Unvested, Ending balance	$ 2.94

Stock-based Compensation Expe_8

Stock-based Compensation Expense - Summary of Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,693	$ 821
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	758	406
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 935	$ 415

Acceleron Collaboration Agree_2

Acceleron Collaboration Agreement - Additional Information (Details) - Acceleron Collaboration Agreement - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	$ 10,000,000		$ 10,000,000
Range of tiered royalties on net sales	mid single-digit percentage to a low double-digit percentage
Change in variable consideration constrained	$ 0
Revenues	800,000	$ 0
Deferred revenue current and non-current	9,600,000		$ 10,000,000
Unbilled accounts receivable related to reimbursable research and development cost	300,000
Research Milestones | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	18,500,000
Clinical and Regulatory Milestones | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	202,500,000
Worldwide Net Sales Milestones | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	$ 217,500,000

Asset Acquisition - Additional

Asset Acquisition - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended
	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
In-process research and development expenses			$ 25,591,000
GSK Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones phase two clinical trial		$ 2,500,000
Direct expenses related to the transaction	$ 100,000
GSK Agreement | Maximum
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones	37,500,000
Specified sales milestone	$ 60,000,000
Series B Convertible Preferred Stock | GSK Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Shares issued for asset acquisition	12,500,000
Shares issued for asset acquisition, estimated fair value	$ 25,500,000
Shares issued, price per share	$ 2.04

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	3 Months Ended
	Mar. 31, 2020USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Nov. 30, 2017USD ($)ft²
Commitments And Contingencies [Line Items]
Total commitment	$ 21,185,000
Indemnification Agreement
Commitments And Contingencies [Line Items]
Accrued liabilities for indemnification agreements	0		$ 0
Legal Proceedings
Commitments And Contingencies [Line Items]
Expenses incurred for legal proceedings	0	$ 0
Cambridge, Massachusetts | Commitment for Office and Laboratory Space Lease of Current Corporate Headquarters
Commitments And Contingencies [Line Items]
Square feet of office and laboratory space leased under operating lease | ft²				28,731
Total commitment				$ 25,100,000
Operating lease term				10 years
Letters of credit				$ 1,100,000
Rent expense	$ 500,000	$ 500,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 1,723
2021	2,354
2022	2,424
2023	2,497
2024	2,572
Thereafter	9,615
Total minimum lease payments	$ 21,185

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details)	3 Months Ended
	Mar. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan name	401(k)

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	3,402,529	18,625,681
Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share		8,571,427
Series B Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share		7,499,991
Outstanding Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	2,832,467	1,528,387
Unvested Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	570,062	1,025,876

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details) - TRV - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Due to related party	$ 0
Common stock shares issued in exchange for services	142,284
Maximum
Related Party Transaction [Line Items]
Due to related party			$ 100,000
Maximum | Consulting Services
Related Party Transaction [Line Items]
Fees paid in exchange for consulting services		$ 100,000