Foghorn Therapeutics (FHTX) 10-Q 4 Dec 20 2020 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Oct. 31, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Registrant Name	Foghorn Therapeutics Inc.
Entity Central Index Key	0001822462
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	DE
Entity Address, State or Province	MA
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Current Reporting Status	No
Entity Interactive Data Current	Yes
Entity Shell Company	false
Trading Symbol	FHTX
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		35,813,580

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 74,620	$ 14,981
Restricted cash	541	541
Prepaid expenses and other current assets	2,603	1,363
Total current assets	77,764	16,885
Property and equipment, net	16,848	2,683
Restricted cash	1,733	1,733
Deferred offering costs	1,970
Other assets	211	11
Operating lease right-of-use assets	43,902	1,030
Total assets	142,428	22,342
Current liabilities:
Accounts payable	3,554	3,439
Accrued expenses	9,970	3,701
Operating lease liabilities	91	1,360
Notes payable, net of discount	5,990	4,152
Deferred revenue	1,696
Total current liabilities	21,301	12,652
Notes payable, net of discount and current portion	8,801	10,960
Operating lease liabilities, net of current portion	56,005	157
Deferred revenue, net of current portion	13,125
Preferred stock warrant liability	114	45
Total liabilities	99,346	23,814
Commitments and contingencies (Note 10)
Convertible preferred stock (Series A-1, A-2 and B), $0.0001 par value; 43,296,288 and 28,629,622 shares authorized at September 30, 2020 and December 31, 2019, respectively; 40,623,413 and 28,615,546 shares issued and outstanding at September 30, 2020 and December 31, 2019, respectively; liquidation preference of $176,841 at September 30, 2020	176,405	86,544
Stockholders' deficit:
Common stock, $0.0001 par value; 62,000,000 and 46,600,000 shares authorized at September 30, 2020 and December 31, 2019, respectively; 6,244,154 and 5,762,745 shares issued and 6,021,177 and 4,870,851 shares outstanding at September 30, 2020 and December 31, 2019, respectively	1
Additional paid-in capital	8,854	6,120
Accumulated deficit	(142,178)	(94,136)
Total stockholders' deficit	(133,323)	(88,016)
Total liabilities, convertible preferred stock and stockholders' deficit	$ 142,428	$ 22,342

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Common Stock, Par Value Per Share	$ 0.0001	$ 0.0001
Common Stock, Shares Authorized	62,000,000	46,600,000
Common Stock, Shares, Issued	6,244,154	5,762,745
Common Stock, Shares, Outstanding	6,021,177	4,870,851
Convertible Preferred Stock [Member]
Convertible Preferred Stock, Par Value Per Share	$ 0.0001	$ 0.0001
Convertible Preferred Stock, Shares Authorized	43,296,288	28,629,622
Convertible Preferred Stock, Shares Issued	40,623,413	28,615,546
Convertible Preferred Stock, Shares Outstanding	40,623,413	28,615,546
Convertible Preferred Stock, Liquidation Preference	$ 176,841

Condensed Consolidated Statemen

Condensed Consolidated Statements Of Operations And Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 179		$ 179
Operating expenses:
Research and development	16,113	$ 11,292	41,244	$ 30,842
General and administrative	2,555	1,808	6,687	5,056
Total operating expenses	18,668	13,100	47,931	35,898
Loss from operations	(18,489)	(13,100)	(47,752)	(35,898)
Other income (expense):
Interest expense	(202)	(113)	(658)	(362)
Interest income and other income (expense), net	394	121	437	424
Change in fair value of preferred stock warrant liability	(70)		(69)
Total other income (expense), net	122	8	(290)	62
Net loss and comprehensive loss	$ (18,367)	$ (13,092)	$ (48,042)	$ (35,836)
Net loss per share attributable to common stockholders—basic and diluted	$ (3.12)	$ (3.05)	$ (8.76)	$ (8.96)
Weighted average common shares outstanding—basic and diluted	5,884,027	4,294,663	5,487,154	4,000,939

Condensed Consolidated Statem_2

Condensed Consolidated Statements Of Convertible Preferred Stock And Stockholders' Deficit - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit	Convertible Preferred Stock [Member]
Beginning balance at Dec. 31, 2018					$ 71,250
Beginning balance (Shares) at Dec. 31, 2018					26,575,544
Issuance of Series B convertible preferred stock, net of issuance costs					$ 15,294
Issuance of Series B convertible preferred stock, net of issuance costs, shares					2,040,002
Ending balance at Sep. 30, 2019					$ 86,544
Ending balance (Shares) at Sep. 30, 2019					28,615,546
Beginning balance at Dec. 31, 2018	$ (39,273)		$ 3,735	$ (43,008)
Beginning balance (Shares) at Dec. 31, 2018		3,475,152
Issuance of common stock upon exercise of stock options	191		191
Issuance of common stock upon exercise of stock options , shares		305,852
Vesting of restricted stock, shares		668,918
Stock-based compensation expense	1,124		1,124
Net loss	(35,836)			(35,836)
Ending balance at Sep. 30, 2019	(73,794)		5,050	(78,844)
Ending balance (Shares) at Sep. 30, 2019		4,449,922
Beginning balance at Dec. 31, 2018					$ 71,250
Beginning balance (Shares) at Dec. 31, 2018					26,575,544
Ending balance at Dec. 31, 2019					$ 86,544
Ending balance (Shares) at Dec. 31, 2019					28,615,546
Beginning balance at Dec. 31, 2018	(39,273)		3,735	(43,008)
Beginning balance (Shares) at Dec. 31, 2018		3,475,152
Net loss	(51,100)
Ending balance at Dec. 31, 2019	$ (88,016)		6,120	(94,136)
Ending balance (Shares) at Dec. 31, 2019	4,870,851	4,870,851
Beginning balance at Jun. 30, 2019					$ 86,544
Beginning balance (Shares) at Jun. 30, 2019					28,615,546
Ending balance at Sep. 30, 2019					$ 86,544
Ending balance (Shares) at Sep. 30, 2019					28,615,546
Beginning balance at Jun. 30, 2019	$ (61,283)		4,469	(65,752)
Beginning balance (Shares) at Jun. 30, 2019		4,014,627
Issuance of common stock upon exercise of stock options	138		138
Issuance of common stock upon exercise of stock options , shares		212,322
Vesting of restricted stock, shares		222,973
Stock-based compensation expense	443		443
Net loss	(13,092)			(13,092)
Ending balance at Sep. 30, 2019	(73,794)		5,050	(78,844)
Ending balance (Shares) at Sep. 30, 2019		4,449,922
Beginning balance at Dec. 31, 2019					$ 86,544
Beginning balance (Shares) at Dec. 31, 2019					28,615,546
Issuance of Series B convertible preferred stock, net of issuance costs					$ 89,861
Issuance of Series B convertible preferred stock, net of issuance costs, shares					12,007,867
Ending balance at Sep. 30, 2020					$ 176,405
Ending balance (Shares) at Sep. 30, 2020					40,623,413
Beginning balance at Dec. 31, 2019	$ (88,016)		6,120	(94,136)
Beginning balance (Shares) at Dec. 31, 2019	4,870,851	4,870,851
Issuance of common stock upon exercise of stock options	$ 852		852
Issuance of common stock upon exercise of stock options , shares		481,409
Vesting of restricted stock		$ 1	(1)
Vesting of restricted stock, shares		668,917
Stock-based compensation expense	1,883		1,883
Net loss	(48,042)			(48,042)
Ending balance at Sep. 30, 2020	$ (133,323)	$ 1	8,854	(142,178)
Ending balance (Shares) at Sep. 30, 2020	6,021,177	6,021,177
Beginning balance at Jun. 30, 2020					$ 134,480
Beginning balance (Shares) at Jun. 30, 2020					35,023,413
Issuance of Series B convertible preferred stock, net of issuance costs					$ 41,925
Issuance of Series B convertible preferred stock, net of issuance costs, shares					5,600,000
Ending balance at Sep. 30, 2020					$ 176,405
Ending balance (Shares) at Sep. 30, 2020					40,623,413
Beginning balance at Jun. 30, 2020	$ (116,411)	$ 1	7,399	(123,811)
Beginning balance (Shares) at Jun. 30, 2020		5,434,649
Issuance of common stock upon exercise of stock options	578		578
Issuance of common stock upon exercise of stock options , shares		363,556
Vesting of restricted stock, shares		222,972
Stock-based compensation expense	877		877
Net loss	(18,367)			(18,367)
Ending balance at Sep. 30, 2020	$ (133,323)	$ 1	$ 8,854	$ (142,178)
Ending balance (Shares) at Sep. 30, 2020	6,021,177	6,021,177

Condensed Consolidated Statem_3

Condensed Consolidated Statements Of Convertible Preferred Stock And Stockholders' Deficit- (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Series B Convertible Preferred Stock [Member]
Stock Issuance Costs	$ 75	$ 198	$ 6

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (48,042)	$ (35,836)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,883	1,124
Depreciation and amortization expense	764	460
Loss on disposal of property and equipment		11
Change in fair value of preferred stock warrant liability	69
Noncash lease expense	3,103	820
Noncash interest expense	179	58
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,725)	(507)
Accounts payable	(374)	559
Accrued expenses and other current liabilities	(198)	942
Operating lease liabilities	8,889	(782)
Deferred revenue	14,821
Net cash used in operating activities	(20,631)	(33,151)
Cash flows from investing activities:
Purchases of property and equipment	(8,694)	(840)
Proceeds from sale of property and equipment		4
Net cash used in investing activities	(8,694)	(836)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	89,861	15,294
Proceeds from issuance of common stock upon exercise of stock options	852	191
Repayment of notes payable	(500)
Payment of initial public offering costs	(1,249)
Payment of notes payable issuance costs		(16)
Net cash provided by financing activities	88,964	15,469
Net increase (decrease) in cash, cash equivalents and restricted cash	59,639	(18,518)
Cash, cash equivalents and restricted cash at beginning of period	17,255	40,585
Cash, cash equivalents and restricted cash at end of period	76,894	22,067
Supplemental cash flow information:
Cash paid for interest	490	$ 312
Supplemental disclosure of noncash investing and financing information:
Purchases of property and equipment included in accounts payable and accrued expenses	6,602
Deferred offering costs included in accounts payable and accrued expenses	$ 721

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 74,620	$ 21,501
Restricted cash (current and non-current)	2,274	566
Cash, cash equivalents and restricted cash at end of period	$ 76,894	$ 22,067

Nature of Business and Basis of

Nature of Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Nature of Business and Basis of Presentation	1. Nature of Business and Basis of Presentation Foghorn Therapeutics Inc. (the “Company”) is a development-stage biopharmaceutical company discovering and developing a new class of medicines targeting genetically determined dependencies within the chromatin regulatory system. The Company uses its proprietary Gene Traffic Control platform to identify, validate and potentially drug targets within the system. The Company was founded in October 2015 as a Delaware corporation. The Company is headquartered in Cambridge, Massachusetts. The Company is subject to risks similar to those of other development-stage companies in the biopharmaceutical industry, including dependence on key individuals, the need to develop commercially viable products, competition from other companies, many of whom are larger and better capitalized, the impact of the COVID-19 In March 2020, the World Health Organization declared the global novel coronavirus disease 2019 (“COVID-19”) outbreak the COVID-19 pandemic. the COVID-19 pandemic the COVID-19 outbreak of COVID-19 on Going concern On October 27, 2020, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold On November 19, 2020, the Company issued and sold an additional 951,837 shares of common stock at the IPO price of $16.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $14.2 million after deducting underwriting discounts and commissions. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations primarily with proceeds from sales of preferred stock, debt financing and an upfront payment of $15.0 million the Company received in July 2020 under a collaboration agreement with Merck Sharp & Dohme Corp., and most recently, with proceeds from the sale of common stock in the IPO completed in October 2020. The Company has incurred recurring losses, including net losses of million for the year ended December 31, 2019. As of September 30, 2020, the Company had an accumulated deficit of million. The Company expects to continue to generate operating losses in the foreseeable future. As of the issuance date of these interim condensed consolidated financial statements the Company expects that the net proceeds it received from its IPO, together with its cash and cash equivalents will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months. The Company will need to obtain additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborative or strategic alliances or licensing arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or programs. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, pipeline expansion or commercialization efforts, which could adversely affect its business prospects. Although management will continue to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations when needed or at all. Basis of presentation The Company’s condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany accounts and transactions have been eliminated in consolidation. 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 Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Unaudited interim financial information The accompanying condensed consolidated balance sheet as of September 30, 2020, the condensed consolidated statements of operations and comprehensive loss, convertible preferred stock and stockholders’ deficit and cash flows for the three and nine months ended September 30, 2020 and 2019 are unaudited. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2020 and the results of its operations for the three and nine months ended September 30, 2020 and 2019 and its cash flows for the nine months ended September 30, 2020 and 2019. The results for the three or nine months ended September 30, 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the valuation of stock-based awards and the accrual of research and development expenses. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. As of September 30, 2020, the Company maintained cash balances in excess of federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to provide services, supplies and materials for certain activities related to its discovery programs. These programs could be adversely affected by a significant interruption in these services or the availability of materials. Deferred offering costs The Company capitalizes certain legal, professional, accounting and other third-party fees that are directly associated with in-process additional paid-in capital condensed Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Furniture and fixtures 5 years Computer equipment and software 3 years Leasehold improvements Shorter of useful life or remaining term of lease Costs for capital assets not yet placed into service are capitalized and depreciated once placed into service. During the nine months ended September 30, 2020, the Company capitalized million of assets not yet placed in service related to leasehold improvements for its new lease. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. Fair value measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and preferred stock warrant liability are carried at fair value, determined according to the fair value hierarchy described above (see Note . The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s long-term debt approximates its fair value (a level measurement) due to its variable interest rate. Revenue recognition The Company accounts for its one collaboration arrangement, entered into in July 2020, under ASC Topic 606, Revenue From Contracts With Customers The Company assesses the goods or services promised within each contract and determines those that are performance obligations. The promised goods or services in the Company’s arrangements would likely consist of licenses, rights to the Company’s intellectual property, research and development services and related supporting activities. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the cumulative catch-up basis If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company records amounts as accounts receivable when the right to consideration is deemed unconditional. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded for deferred revenue. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between the transfer of the promised goods or services to the customer and the payment by the customer will be one year or less. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied, either at a point in time or over time, and if over time, recognition is based on the use of an output or input method. Net loss per share The Company follows the two-class method The two-class method The two-class method Basic net income (loss) per share attributable to common stockholders is computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. The Company’s participating securities contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the three and nine months ended September 30, 2020 and 2019. The following common stock equivalents presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: September 30, 2020 2019 Convertible preferred stock (as converted to common stock) 21,958,588 15,467,863 Warrants to purchase convertible preferred stock (as converted to common stock) 7,608 7,608 Unvested restricted common stock 222,977 1,114,867 Stock options to purchase common stock 4,931,761 4,084,445 27,120,934 20,674,783 Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at September 30, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 74,521 $ — $ — $ 74,521 Liabilities: Preferred stock warrant liability $ — $ — $ 114 $ 114 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 14,951 $ — $ — $ 14,951 Liabilities: Preferred stock warrant liability $ — $ — $ 45 $ 45 During the three and nine months ended September 30, 2020, there were no transfers between Level 1, Level 2 and Level 3. The preferred stock warrant liability in the tables above consisted of the fair value of warrants to purchase 14,076 shares of Series A-1 condensed The quantitative elements associated with the Company’s Level 3 inputs impacting the fair value measurement of the preferred stock warrant liability include the fair value per share of the underlying Series A-1 A-1 A-1

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2020
Payables and Accruals [Abstract]
Accrued Expenses	4. Accrued Expenses Accrued expenses consisted of the following (in thousands): September 30, 2020 December 31, 2019 Accrued construction in progress $ 6,208 $ 119 Accrued employee compensation and benefits 2,086 1,867 Accrued external research and development expenses 1,089 1,384 Other 587 331 $ 9,970 $ 3,701

Notes Payable

Notes Payable	9 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Notes Payable	5. Notes Payable Long-term debt consisted of the following (in thousands): September 30, 2020 December 31, 2019 Principal amount of long-term debt $ 14,500 $ 15,000 Less: Current portion of long-term debt (5,990 ) (4,152 ) Long-term debt, net of current portion 8,510 10,848 Final payment fee 530 530 Debt discount, net of accretion (239 ) (418 ) Long-term debt, net of discount and current portion $ 8,801 $ 10,960 The Company has outstanding loans under its amended loan and security agreement (the “Loan”) of $6.8 million (“Term Loan A”) and $7.7 million (“Term Loan B”) so that the total amount outstanding under the Loan was $14.5 million as of September 30, 2020. Borrowings under both Term Loan A and Term Loan B were repayable in monthly payments of interest-only through February 2020, to be followed by monthly payments of equal principal plus interest until the loan maturity date of February 1, 2023. In April 2020, the Company amended the Loan to extend the interest only period through May 31, 2020 and in June 2020 the Loan was further amended to extend the interest-only period through August 31, 2020. Interest for Term Loan A is the greater of 1) Comerica’s Prime Rate or 2) LIBOR plus 2.5%, and for Term Loan B, 1.0% plus the greater of 1) Comerica’s Prime Rate or 2) LIBOR plus 2.5%. A final payment fee of 3.0% of the aggregate amounts drawn under Term Loan A and 4.0% under Term Loan B is due upon the earlier of the maturity date, the repayment date if paid early, whether voluntary or upon acceleration due to default, the sale of substantially all of the Company’s assets, or the Company’s IPO. The Company may repay the Loan at any time by paying the outstanding principal balance in full, along with any unpaid accrued interest and the final payment fee. The final payment fee of $0.5 million is being amortized to interest expense over the term of the debt using the effective interest method. Upon closing of the Company’s IPO in October 2020, the final payment fee became due. Borrowings under the Loan are collateralized by substantially all of the Company’s assets, other than its intellectual property. There are no financial covenants associated with the Loan; however, the Company is subject to certain affirmative and negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions; encumbering its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and engaging in certain other business transactions. The obligations under the Loan are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. Upon the occurrence of an event of default and until such event of default is no longer continuing, the annual interest rate will be above the otherwise applicable rate. As of September 30, 2020 and December 31, 2019, the Company believes an event of default would be remote. As of September 30, 2020, the interest rate applicable to outstanding borrowings under the Loan was 3.8%. During the three and nine months ended September 30, 2020, the weighted average effective interest rate on outstanding borrowings under the Loan was approximately 5.5% and 5.9%, respectively. As of September 30, 2020, future principal payments due are as follows (in thousands): Remainder of 2020 (three months) $ 1,500 2021 6,000 2022 6,000 2023 1,000 $ 14,500 In November 2020, the Company entered into a term loan agreement for $20.0 million with a new lender and used proceeds from

Convertible Preferred Stock

Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2020
Stockholders' Equity Note [Abstract]
Convertible Preferred Stock	6. Convertible Preferred Stock The Company has issued Series A-1 convertible (the “Series A-1”), Series A-2 convertible “Series A-2”) and The Series A-1 and Series A-2 are In April 2020, in two separate closings, the Company sold 6,407,867 shares of Series B preferred stock at a purchase price of $7.50 per share resulting in gross proceeds to the Company of $48.1 million. In July and August 2020, in two separate closings, the Company sold 5,600,000 shares of Series B preferred stock at a purchase price of $7.50 per share resulting in gross proceeds to the Company of $42.0 million. Upon the closing of the IPO in October 2020 (see Note 13), the Company’s Preferred Stock automatically converted into 21,958,588 shares of common stock.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation 2016 Stock incentive plan The Company grants stock-based awards under its 2016 Stock Incentive Plan, (the “2016 Plan”). The total number of shares of common stock that may be issued under the 2016 Plan was 6,513,512 shares as of September 30, 2020, of which 227,354 shares remained available for future grant as of September 30, 2020. Upon the effectiveness of the 2020 Equity Incentive Plan (the “2020 Plan”) in October 2020 (see Note 13), the Company ceased granting additional awards under the 2016 Plan. Stock-based compensation Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development expenses $ 500 $ 292 $ 1,125 $ 747 General and administrative expenses 377 151 758 377 $ 877 $ 443 $ 1,883 $ 1,124 As of September 30, 2020, total unrecognized compensation cost related to unvested options and unvested restricted stock was $12.1 million, which is expected to be recognized over a weighted average period of 3.3 years.

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2020
Disclosure Text Block [Abstract]
Collaboration Agreement	8. Collaboration Agreement In July 2020, the Company entered into a research collaboration and license agreement (the “Collaboration Agreement”) with Merck Sharp & Dohme Corp. (“Merck”). The Company and Merck will apply Foghorn’s proprietary Gene Traffic Control platform to discover and develop novel therapeutics. Under the Collaboration Agreement, the Company granted Merck exclusive global rights to develop and commercialize drugs that target dysregulation of a single transcription factor. Under the terms of the Collaboration Agreement, the Company and Merck are each responsible to perform certain research activities in accordance with a mutually agreed upon research plan. Merck may substitute the selected transcription factor during certain stages of the research program, subject to certain limitations. Following completion of the research program, Merck is responsible for the development and commercialization of the compounds developed pursuant to the research program and any product containing such compounds. Pursuant to the Collaboration Agreement, the Company will also participate on a joint steering committee. Under the terms of the agreement, Foghorn received a nonrefundable upfront payment of $ million from Merck, and is eligible to receive up to $ million upon first achievement of specified research, development and regulatory milestones by any product candidate generated by the collaboration, and up to $ million upon achievement of specified sales-based milestones per approved product from the collaboration, if any. The Company will be eligible to receive tiered royalties, calculated on a product-by-product country-by-country Unless terminated earlier, the Collaboration Agreement will continue in full force and effect until one or more products has received marketing authorization and, thereafter, until expiration of all royalty obligations under the Collaboration Agreement. The Company or Merck may terminate the Collaboration Agreement upon an uncured material breach by the other party or insolvency of the other party. Merck may also terminate the Merck Collaboration Agreement for any reason upon certain notice to the Company. The Company determined that the (1) research, development and commercialization licenses, (2) the research activities performed by the Company and (3) service on the joint committees represent a single performance obligation under the Collaboration Agreement. The Company determined that Merck cannot benefit from licenses separately from the research activities and participation on the joint steering committee as these services are specialized and rely on the Company’s expertise such that these activities are highly interrelated and therefore not distinct. Accordingly, the promised goods and services represent one combined performance obligation and the entire transaction price will be allocated to that single combined performance obligation. The performance obligation will be satisfied over the research term as the Company performs the research activities and participates in a joint steering committee to oversee research activities. Accordingly, the upfront payment of $15.0 million was initially recorded as deferred revenue and is being recognized as revenue as the performance obligation is satisfied. The Company recognizes revenue using the cost-to-cost method, which Under the cost-to-cost method, the revenue catch-up As of September 30, 2020, the aggregate amount of the transaction price related to the unsatisfied portion of the performance obligation is $14.8 million, which is expected to be recognized as revenue through 2028. The Company does not expect collaboration revenue to be recognized evenly over this period as it will be recognized on a percentage of completion basis (using cost-to-cost The Company assessed the Collaboration Agreement to determine whether a significant financing component exists and concluded that a significant financing component does not exist. Through September 30, 2020, the Company had recorded $0.2 million of revenue under the Collaboration Agreement.

Leases

Leases	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Leases	9. Leases The Company has a lease for office and laboratory facilities in Cambridge, Massachusetts (the “Existing Lease”) under a noncancelable operating lease that began in August 2017 and expires in March 2025. In April 2020, the Existing Lease was assigned and assumed by a related party which became effective in October 2020. In October 2019, the Company entered into a lease for 81,441 square feet of office and laboratory space in Cambridge, Massachusetts, commencing in January 2020 (the “New Lease”). The initial term of the New Lease was eight years with a five-year option to extend at fair-market rent at the time of the extension. The base rent payments escalate annually over the eight-year lease term and totaled approximately $60.3 million. In connection with the New Lease, the landlord agreed to fund up to $3.0 million in tenant improvements to the leased facility as well as up to an additional $16.3 million, which will result in additional rent payments to the landlord. During the three and nine months ended September 30, 2020, $5.4 million and $8.4 million, respectively, of leasehold improvements were reimbursed by the landlord, which resulted in an increase to operating lease liabilities. The Company will be obligated to pay its portion of real estate taxes and costs related to the premises, including costs of operations and management of the leased premises. On January 1, 2020, the lease commencement date, the Company recorded an operating lease asset of $38.6 million and corresponding lease liability of $38.3 million. In June 2020, the Company amended the New Lease to defer payment of a portion of the base rent and operating expenses and to extend the lease term by nine months to September 2028. The amendment was accounted for as a lease modification and the right-of- right-of-use The components of lease expense were as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Operating lease cost $ 2,162 $ 445 $ 5,145 $ 1,318 Short-term lease cost — 23 46 38 Variable lease cost 225 110 617 348 $ 2,387 $ 578 $ 5,808 $ 1,704 Supplemental disclosure of cash flow information related to leases was as follows (in thousands): Nine Months Ended September 30, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 1,571 $ 1,280 Operating lease liabilities arising from obtaining right-of-use $ 38,306 $ 271 Increase in operating lease liabilities and right-of-use $ 7,384 $ — The weighted-average remaining lease term and discount rate as of period ends were as follows: September 30, 2020 December 31, 2019 Weighted-average remaining lease term - operating leases (in years) 7.91 1.06 Weighted-average discount rate - operating leases 5.35 % 7.53 % Future annual minimum lease payments under the Company’s operating leases as of September 30, 2020 were as follows (in thousands): Remainder of 2020 (three months) $ 1,517 2021 9,804 2022 10,015 2023 10,108 2024 10,349 Thereafter 41,100 Total future minimum lease payments 82,893 Less: imputed interest (15,891 ) Less: estimated lease incentives (10,906 ) Total operating lease liabilities $ 56,096 Included in the consolidated balance sheet (in thousands): September 30, 2020 Current operating lease liabilities $ 91 Operating lease liabilities, net of current portion 56,005 Total operating lease liabilities $ 56,096 Sublease agreement In April 2020, the Company entered into a two-year

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Leases The Company’s commitments under its leases are described in Note 9. License agreements Dana-Farber Cancer Institute In 2016, the Company entered into a license agreement with the Dana-Farber Cancer Institute, Inc. (“Dana Farber”) for an exclusive license for certain biological materials as well as patent rights to methods of identifying compounds to treat prostate cancer. In consideration for the right to develop, manufacture, and commercialize products based on certain of Dana Farber’s intellectual property, the Company is obligated to reimburse Dana Farber for patent expenses and pay low single-digit sales-based royalties upon the occurrence of specific events as outlined in the license agreement. Unless terminated earlier, in accordance with the provisions of the agreement, the agreement will terminate on the expiration date of the last to expire of the applicable Dana Farber patents. None of the Company’s product candidates utilize technology covered by this license. Stanford In July 2017, the Company entered into a license agreement with the Board of Trustees of the Leland Stanford Junior University (“Stanford”) for a non-exclusive one-time, non-refundable Indemnification agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal Proceedings From time to time, the Company may become involved in litigation or other legal proceedings. The Company is not currently a party to any material litigation or legal proceedings.

Defined Contribution Plan

Defined Contribution Plan	9 Months Ended
	Sep. 30, 2020
Retirement Benefits [Abstract]
Defined Contribution Plan	11. Defined Contribution Plan The Company has a 401(k) defined contribution plan (the “401(k) Plan”) for its employees. Eligible employees may make pretax contributions to the 401(k) Plan up to statutory limits. There has been no discretionary match made under the 401(k) Plan as of September 30, 2020.

Related Parties

Related Parties	9 Months Ended
	Sep. 30, 2020
Related Party Transactions [Abstract]
Related Parties	12. Related Parties In October 2015, the Company entered into a five-year service agreement with Flagship Pioneering (“Flagship”), an affiliate of one of its stockholders , As of September 30, 2020, the Company had less than $0.1 million in accounts payable to Flagship for costs related to the service agreement. In October 2015, the Company entered into a five-year consulting agreement with a scientific founder of the Company who is also a board member and a shareholder. During the three and nine months ended September 30, 2020, the Company paid the scientific founder million and million, respectively. During the three and nine months ended September 30, 2019, the Company paid the scientific founder $0.1 million and $0.2 million, respectively. As of September 30, 2020 and December 31, 2019, the Company had no accounts payable to this scientific founder.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2020
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events On October 18, 2020, the board of directors of the Company approved a 1-for-1.85 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the conversion ratios for each series of the Company’s outstanding Preferred Stock. The reverse stock split legally occurred upon the filing of the certificate of amendment to the Company’s amended and restated certification of incorporation, which was filed with the State of Delaware on October 21, 2020. All share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the P S 2020 Equity Incentive Plan On October 21, 2020, the Company’s board of directors adopted and its stockholders approved the 2020 Plan, which became effective on October 21, 2020. The 2020 Plan provides for the grant of incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. 2020 Employee Stock Purchase Plan On October 21, 2020, the Company’s board of directors adopted and its stockholders approved the 2020 Employee Stock Purchase Plan (the “ESPP”), which became effective on October 21, 2020. Changes to Authorized Common and Preferred Shares On October 27, 2020, the Company filed a restated certificate Initial Public Offering On October 27, 2020, the Company completed its IPO pursuant million, after deducting underwriting discounts and commissions and other offering expenses. On November 19, 2020, the Company issued and sold an additional 951,837 shares of common stock at the IPO price of $16.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $14.2 million after deducting underwriting discounts and commissions. Upon the closing of the IPO, all of the Company’s outstanding convertible preferred stock automatically converted into shares of common stock (see Note 6). Upon the closing of the IPO, the Company’s outstanding warrants to purchase Series A Preferred Stock automatically became warrants to purchase an aggregate of 7,608 shares of common stock. Loan and Security Agreements On November 19, 2020, the Company entered into a new loan and security agreement, or the Oxford Loan, with Oxford Finance LLC, or Oxford, for (a) thirty-day %. In addition, upon loan maturity or prepayment, the Company is reqired to make a final payment fee equal to 5.0% of the aggregate principal amount borrowed. The Company is required to make monthly interest only payments under the Oxford Loan on the first calendar day of each month beginning on January 1, 2021. Beginning on December 1, 2023, the Company is required to make consecutive equal monthly payments of principal, together with applicable interest, in arrears, based upon a repayment schedule equal to 24 months, with a final maturity date of November 1, 2025. The Company’s obligations under the Oxford Loan Agreement will be secured by a security interest in all of its assets, other than its intellectual property. The Company is also subject to certain affirmative and negative covenants. In connection with the Oxford Loan Agreement, the Company granted Oxford warrants to purchase 18,445 shares of the Company’s common stock at $16.26 per share. The issued warrants are exerciseable for 10 years. On November 19, 2020, the Company borrowed $20.0 million under the Oxford Term Loan A and used a portion of the proceeds to repay the outstanding principal balance plus unpaid accrued interest and the final payment fee related to the loan and security agreement with Comerica.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The Company’s condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany accounts and transactions have been eliminated in consolidation. 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 Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Unaudited interim financial information The accompanying condensed consolidated balance sheet as of September 30, 2020, the condensed consolidated statements of operations and comprehensive loss, convertible preferred stock and stockholders’ deficit and cash flows for the three and nine months ended September 30, 2020 and 2019 are unaudited. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2020 and the results of its operations for the three and nine months ended September 30, 2020 and 2019 and its cash flows for the nine months ended September 30, 2020 and 2019. The results for the three or nine months ended September 30, 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.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the valuation of stock-based awards and the accrual of research and development expenses. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Concentrations of credit risk and of significant suppliers	Concentrations of credit risk and of significant suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. As of September 30, 2020, the Company maintained cash balances in excess of federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to provide services, supplies and materials for certain activities related to its discovery programs. These programs could be adversely affected by a significant interruption in these services or the availability of materials.
Deferred offering costs	Deferred offering costs The Company capitalizes certain legal, professional, accounting and other third-party fees that are directly associated with in-process additional paid-in capital condensed
Property and equipment	Property and equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Furniture and fixtures 5 years Computer equipment and software 3 years Leasehold improvements Shorter of useful life or remaining term of lease Costs for capital assets not yet placed into service are capitalized and depreciated once placed into service. During the nine months ended September 30, 2020, the Company capitalized million of assets not yet placed in service related to leasehold improvements for its new lease. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.
Fair value measurements	Fair value measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and preferred stock warrant liability are carried at fair value, determined according to the fair value hierarchy described above (see Note . The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s long-term debt approximates its fair value (a level measurement) due to its variable interest rate.
Revenue recognition	Revenue recognition The Company accounts for its one collaboration arrangement, entered into in July 2020, under ASC Topic 606, Revenue From Contracts With Customers The Company assesses the goods or services promised within each contract and determines those that are performance obligations. The promised goods or services in the Company’s arrangements would likely consist of licenses, rights to the Company’s intellectual property, research and development services and related supporting activities. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the cumulative catch-up basis If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company records amounts as accounts receivable when the right to consideration is deemed unconditional. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded for deferred revenue. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between the transfer of the promised goods or services to the customer and the payment by the customer will be one year or less. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied, either at a point in time or over time, and if over time, recognition is based on the use of an output or input method.
Net loss per share	Net loss per share The Company follows the two-class method The two-class method The two-class method Basic net income (loss) per share attributable to common stockholders is computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net income (loss) per share attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares. The Company’s participating securities contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the three and nine months ended September 30, 2020 and 2019. The following common stock equivalents presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: September 30, 2020 2019 Convertible preferred stock (as converted to common stock) 21,958,588 15,467,863 Warrants to purchase convertible preferred stock (as converted to common stock) 7,608 7,608 Unvested restricted common stock 222,977 1,114,867 Stock options to purchase common stock 4,931,761 4,084,445 27,120,934 20,674,783
Recently issued accounting pronouncements	Recently issued accounting pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies that the Company adopts as of the specified effective date. The Company qualifies as an “emerging growth company” as defined in the Jumpstart Our Business Startups Act of 2012 and has elected not to “opt out” of the extended transition related to complying with new or revised accounting standards, which means that when a standard is issued or revised and it has different application dates for public and nonpublic companies, the Company can adopt the new or revised standard at the time nonpublic companies adopt the new or revised standard and can do so until such time that the Company either (i) irrevocably elects to “opt out” of such extended transition period or (ii) no longer qualifies as an emerging growth company.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Summary of Property and Equipment are Stated at Cost Less Accumulated Depreciation and Amortization	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Furniture and fixtures 5 years Computer equipment and software 3 years Leasehold improvements Shorter of useful life or remaining term of lease
Summary of Earnings Per Share Diluted Anti Dilutive Impact	The following common stock equivalents presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: September 30, 2020 2019 Convertible preferred stock (as converted to common stock) 21,958,588 15,467,863 Warrants to purchase convertible preferred stock (as converted to common stock) 7,608 7,608 Unvested restricted common stock 222,977 1,114,867 Stock options to purchase common stock 4,931,761 4,084,445 27,120,934 20,674,783

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at September 30, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 74,521 $ — $ — $ 74,521 Liabilities: Preferred stock warrant liability $ — $ — $ 114 $ 114 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 14,951 $ — $ — $ 14,951 Liabilities: Preferred stock warrant liability $ — $ — $ 45 $ 45

Accrued Expenses (Table)

Accrued Expenses (Table)	9 Months Ended
	Sep. 30, 2020
Payables and Accruals [Abstract]
Accrued Expenses	Accrued expenses consisted of the following (in thousands): September 30, 2020 December 31, 2019 Accrued construction in progress $ 6,208 $ 119 Accrued employee compensation and benefits 2,086 1,867 Accrued external research and development expenses 1,089 1,384 Other 587 331 $ 9,970 $ 3,701

Notes Payable (Tables)

Notes Payable (Tables)	9 Months Ended
	Sep. 30, 2020
Long-term Debt, Unclassified [Abstract]
Summary of Long Term Debt	Long-term debt consisted of the following (in thousands): September 30, 2020 December 31, 2019 Principal amount of long-term debt $ 14,500 $ 15,000 Less: Current portion of long-term debt (5,990 ) (4,152 ) Long-term debt, net of current portion 8,510 10,848 Final payment fee 530 530 Debt discount, net of accretion (239 ) (418 ) Long-term debt, net of discount and current portion $ 8,801 $ 10,960
Summary of Future Long Term Loan Principal Payments Due	As of September 30, 2020, future principal payments due are as follows (in thousands): Remainder of 2020 (three months) $ 1,500 2021 6,000 2022 6,000 2023 1,000 $ 14,500

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Summary of Stock-Based Compensation Expense	Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Research and development expenses $ 500 $ 292 $ 1,125 $ 747 General and administrative expenses 377 151 758 377 $ 877 $ 443 $ 1,883 $ 1,124

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Summary of Lease Cost	The components of lease expense were as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Operating lease cost $ 2,162 $ 445 $ 5,145 $ 1,318 Short-term lease cost — 23 46 38 Variable lease cost 225 110 617 348 $ 2,387 $ 578 $ 5,808 $ 1,704
Summary of Supplemental Cash Flow Information Related to Leases	Supplemental disclosure of cash flow information related to leases was as follows (in thousands): Nine Months Ended September 30, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 1,571 $ 1,280 Operating lease liabilities arising from obtaining right-of-use $ 38,306 $ 271 Increase in operating lease liabilities and right-of-use $ 7,384 $ —
Summary of Weighted-Average Remaining Lease Term and Discount Rate Leases	The weighted-average remaining lease term and discount rate as of period ends were as follows: September 30, 2020 December 31, 2019 Weighted-average remaining lease term - operating leases (in years) 7.91 1.06 Weighted-average discount rate - operating leases 5.35 % 7.53 %
Summary of Minimum Lease Payments	Future annual minimum lease payments under the Company’s operating leases as of September 30, 2020 were as follows (in thousands): Remainder of 2020 (three months) $ 1,517 2021 9,804 2022 10,015 2023 10,108 2024 10,349 Thereafter 41,100 Total future minimum lease payments 82,893 Less: imputed interest (15,891 ) Less: estimated lease incentives (10,906 ) Total operating lease liabilities $ 56,096
Summary of Operating Lease Liabilities	Included in the consolidated balance sheet (in thousands): September 30, 2020 Current operating lease liabilities $ 91 Operating lease liabilities, net of current portion 56,005 Total operating lease liabilities $ 56,096

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Nov. 19, 2020	Oct. 27, 2020	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net income (loss)				$ (18,367)	$ (13,092)	$ (48,042)	$ (35,836)	$ (51,100)
Retained earnings accumulated deficit				$ (142,178)		(142,178)		$ (94,136)
Upfront payment received from Merck			$ 15,000			$ 15,000
IPO [Member] | Subsequent Event [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during the period shares new issues shares		7,500,000
Sale of stock issue price per share		$ 16
Sale of stock consideration received on the transaction		$ 108,000
Over-Allotment Option [Member] | Subsequent Event [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during the period shares new issues shares	951,837
Sale of stock issue price per share	$ 16
Sale of stock consideration received on the transaction	$ 14,200

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Capitalized costs	$ 14.5
Deferred offering costs	$ 2	$ 0

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Property and Equipment are Stated at Cost Less Accumulated Depreciation and Amortization (Detail)	9 Months Ended
	Sep. 30, 2020
Laboratory equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	Shorter of useful life or remaining term of lease

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Earnings Per Share Diluted Anti Dilutive Impact (Detail) - shares	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	27,120,934	20,674,783
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	21,958,588	15,467,863
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	7,608	7,608
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	222,977	1,114,867
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	4,931,761	4,084,445

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Preferred Stock Warrant Liability [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Preferred stock warrant liability	$ 114	$ 45
Money Market Funds [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Money market funds	74,521	14,951
Level 1 [Member] | Fair Value, Recurring [Member] | Money Market Funds [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Money market funds	74,521	14,951
Level 3 [Member] | Fair Value, Recurring [Member] | Preferred Stock Warrant Liability [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Preferred stock warrant liability	$ 114	$ 45

Fair Value Measurements - Addi

Fair Value Measurements - Additional Information (Detail) - Preferred Stock Warrant Liability [Member]	9 Months Ended
	Sep. 30, 2020$ / sharesshares
Class of warrant or right, number of securities called by warrants or rights | shares	14,076
Warrant issue price | $ / shares	$ 1

Accrued Expenses (Detail)

Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued construction in progress	$ 6,208	$ 119
Accrued employee compensation and benefits	2,086	1,867
Accrued external research and development expenses	1,089	1,384
Other	587	331
Total	$ 9,970	$ 3,701

Notes Payable - Summary of Long

Notes Payable - Summary of Long Term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Long-term Debt, Unclassified [Abstract]
Principal amount of long-term debt	$ 14,500	$ 15,000
Less: Current portion of long-term debt	(5,990)	(4,152)
Long-term debt, net of current portion	8,510	10,848
Final payment fee	530	530
Debt discount, net of accretion	(239)	(418)
Long-term debt, net of discount and current portion	$ 8,801	$ 10,960

Notes Payable - Summary of Futu

Notes Payable - Summary of Future Principal Payments Due (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Long-term Debt, Fiscal Year Maturity [Abstract]
Remainder of 2020 (three months)	$ 1,500
2021	6,000
2022	6,000
2023	1,000
Long-term Debt, Gross	$ 14,500	$ 15,000

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($) $ in Thousands	Nov. 19, 2020	Sep. 30, 2020	Sep. 30, 2020	Nov. 30, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Debt instrument carrying amount		$ 14,500	$ 14,500		$ 15,000
Final payment fee		$ 530	$ 530		$ 530
Interest rate outstanding borrowings loan		3.80%	3.80%
Debt Instrument, Covenant Description			There are no financial covenants associated with the Loan; however, the Company is subject to certain affirmative and negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions; encumbering its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and engaging in certain other business transactions. The obligations under the Loan are subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. Upon the occurrence of an event of default and until such event of default is no longer continuing, the annual interest rate will be 5.0% above the otherwise applicable rate.
Debt, weighted average interest rate		5.50%	5.90%
Oxford Finance LLC [Member] | Subsequent Event [Member]
Short-term Debt [Line Items]
Debt, final payment fee percentage	5.00%
Minimum [Member] | Oxford Finance LLC [Member] | Subsequent Event [Member]
Short-term Debt [Line Items]
Debt instrument, description of variable rate basis	(a) thirty-day U.S. DOLLAR LIBOR rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, plus (b) 7.84%
Debt instrument, basis spread on variable rate	7.84%
Additional Rate In Event Of Default [Member]
Short-term Debt [Line Items]
Debt instrument, interest rate, effective percentage		5.00%	5.00%
Term Loan A [Member]
Short-term Debt [Line Items]
Debt instrument carrying amount		$ 6,800	$ 6,800
Debt, final payment fee percentage		3.00%	3.00%
Term Loan A [Member] | Oxford Finance LLC [Member] | Subsequent Event [Member]
Short-term Debt [Line Items]
Debt instrument carrying amount	$ 20,000			$ 20,000
Term Loan A [Member] | Minimum [Member]
Short-term Debt [Line Items]
Debt instrument, description of variable rate basis			LIBOR
Debt instrument, basis spread on variable rate			2.50%
Term Loan B [Member]
Short-term Debt [Line Items]
Debt instrument carrying amount		$ 7,700	$ 7,700
Debt instrument, interest rate, stated percentage		1.00%	1.00%
Debt, final payment fee percentage		4.00%	4.00%
Term Loan B [Member] | Additional Over Base Rate [Member] | Minimum [Member]
Short-term Debt [Line Items]
Debt instrument, description of variable rate basis			LIBOR
Debt instrument, basis spread on variable rate			2.50%

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	Oct. 31, 2020	Apr. 30, 2020	Aug. 31, 2020
Conversion of preferred stock, common shares issued	21,958,588
Series B [Member]
Sale of preferred stock		$ 6,407,867	$ 5,600,000
Preferred stock, par or stated value per share		$ 7.50	$ 7.50
Proceeds from issuance of convertible preferred stock		$ 48,100,000	$ 42,000,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-based compensation expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock-based compensation expense	$ 877	$ 443	$ 1,883	$ 1,124
Research and Development Expenses [Member]
Stock-based compensation expense	500	292	1,125	747
General and Administrative Expenses [Member]
Stock-based compensation expense	$ 377	$ 151	$ 758	$ 377

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2020USD ($)shares
Share-based Payment Arrangement [Line Items]
Unrecognized compensation costs related to unvested options and unvested restricted stock | $	$ 12.1
Weighted average period of unrecognized compensation costs	3 years 3 months 18 days
2016 Stock incentive plan [Member]
Share-based Payment Arrangement [Line Items]
Number of shares authorized	6,513,512
Common stock shares available for grant	227,354

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2020
Upfront payment received from Merck	$ 15,000		$ 15,000
Deferred revenue		$ 15,000	15,000
Revenue		179	179
Unsatisfied portion of the performance obligation		$ 14,800	14,800
Maximum [Member] | Sales based [Member]
Amount receivable for achievement of milestones			165,000
Research Development And Regulatory [Member] | Maximum [Member]
Amount receivable for achievement of milestones			245,000
Merck Collaboration Agreement [Member]
Revenue			$ 200

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	Jun. 29, 2020USD ($)	Apr. 30, 2020ft²	Oct. 31, 2019USD ($)ft²	Sep. 30, 2020USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2027USD ($)	Jan. 01, 2020USD ($)	Dec. 31, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				$ 43,902	$ 43,902			$ 1,030
Increase in operating lease liabilities and right-of-use assets due to lease remeasurement					7,384
Operating lease liability				56,096	56,096
New Lease Agreement [Member] | Massachusetts [Member] | Office Building [Member]
Lessee, Lease, Description [Line Items]
Area of leased property | ft²			81,441
Operating lease term lessee gross			8 years
Operating lease term extension			5 years
Reimbursements from lessor				5,400	8,400
Operating lease right-of-use assets							$ 38,600
Operating lease liability							$ 38,300
Lease term commencement date			Jan. 1, 2020
New Lease Agreement [Member] | Massachusetts [Member] | Office Building [Member] | Maximum [Member]
Lessee, Lease, Description [Line Items]
Leasehold improvements landlord allowance			$ 3,000
Leasehold improvements landlord allowance to be repaid			$ 16,300
New Lease Agreement [Member] | Massachusetts [Member] | Office Building [Member] | Forecast [Member]
Lessee, Lease, Description [Line Items]
Operating lease rent payments						$ 60,300
Modified New Lease Agreement [Member] | Massachusetts [Member] | Office Building [Member]
Lessee, Lease, Description [Line Items]
Increase in operating lease liabilities and right-of-use assets due to lease remeasurement	$ 7,400
Property Under Operating Lease Subleased [Member] | Massachusetts [Member] | Office Building [Member]
Lessee, Lease, Description [Line Items]
Area of leased property | ft²		16,843
Term of property subleased under operating lease		2 years
Operating lease sublease rental income					3,000
Sublease income recorded as other income				$ 400	$ 400

Leases - Summary of Lease Cost

Leases - Summary of Lease Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Leases [Abstract]
Operating lease cost	$ 2,162	$ 445	$ 5,145	$ 1,318
Short-term lease cost		23	46	38
Variable lease cost	225	110	617	348
Total	$ 2,387	$ 578	$ 5,808	$ 1,704

Leases - Summary of Supplementa

Leases - Summary of Supplemental Cash Flow Information Related to Leases (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Leases [Abstract]
Cash paid for amounts included in the measurement of operating lease liabilities	$ 1,571	$ 1,280
Operating lease liabilities arising from obtaining right-of-use assets	38,306	$ 271
Increase in operating lease liabilities and right-of-use assets due to lease remeasurement	$ 7,384

Leases - Summary of Weighted-A

Leases - Summary of Weighted-Average Remaining Lease Term and Discount Rate Leases (Detail)	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Weighted Average Remaining Lease Term And Discount Rate [Abstract]
Weighted-average remaining lease term - operating leases	7 years 10 months 27 days	1 year 21 days
Weighted-average discount rate - operating leases	5.35%	7.53%

Leases - Summary of Minimum Lea

Leases - Summary of Minimum Lease Payment (Detail) $ in Thousands	Sep. 30, 2020USD ($)
Leases [Abstract]
Remainder of 2020 (three months)	$ 1,517
2021	9,804
2022	10,015
2023	10,108
2024	10,349
Thereafter	41,100
Total future minimum lease payments	82,893
Less: imputed interest	(15,891)
Less: estimated lease incentives	(10,906)
Total operating lease liabilities	$ 56,096

Leases - Summary of Operating

Leases - Summary of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Operating Lease, Liability [Abstract]
Current operating lease liabilities	$ 91	$ 1,360
Operating lease liabilities, net of current portion	56,005	$ 157
Total operating lease liabilities	$ 56,096

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Jul. 31, 2017	Sep. 30, 2020	Dec. 31, 2019
Shares issued under agreement		6,244,154	5,762,745
Stanford License Agreement [Member]
Reimbursed patent costs	$ 0.1
Shares issued under agreement	42,781
Stanford License Agreement [Member] | Maximum [Member]
Non-refundable license fee	$ 0.1
Annual license maintenance fees	0.1
Milestones payable upon event	$ 1.1

Related Parties - Additional In

Related Parties - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2015	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Accounts payable		$ 3,554		$ 3,554		$ 3,439
Flagship Pioneering [Member]
Related Party Transaction [Line Items]
Service agreement term	5 years
Payments made to related party under service agreement		300	$ 200	900	$ 600
Flagship Pioneering [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Accounts payable		100		100		0
Scientific Founder [Member]
Related Party Transaction [Line Items]
Accounts payable		0		0		$ 0
Service agreement term	5 years
Payments made to related party under service agreement		$ 100	$ 100	$ 200	$ 200

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Nov. 19, 2020	Oct. 27, 2020	Oct. 21, 2020	Oct. 18, 2020	Sep. 30, 2020	Nov. 30, 2020	Dec. 31, 2019
Subsequent Event [Line Items]
Common stock, par value per share					$ 0.0001		$ 0.0001
Common stock, shares authorized					62,000,000		46,600,000
Debt instrument loan amount					$ 14,500		$ 15,000
Oxford Finance LLC [Member]
Subsequent Event [Line Items]
Warrants granted to purchase common stock					18,445
Warrants Issue Price Per Share					$ 16.26
Warrants exercisable term					10 years
Term Loan A [Member]
Subsequent Event [Line Items]
Debt instrument loan amount					$ 6,800
Debt, final payment fee percentage					3.00%
Term Loan B [Member]
Subsequent Event [Line Items]
Debt instrument loan amount					$ 7,700
Debt, final payment fee percentage					4.00%
Minimum [Member] | Term Loan A [Member]
Subsequent Event [Line Items]
Debt instrument, basis spread on variable rate					2.50%
Debt instrument, description of variable rate basis					LIBOR
Subsequent Event [Member]
Subsequent Event [Line Items]
Reverse stock split				1-for-1.85
Common stock, par value per share		$ 0.0001
Preferred stock, shares authorized		25,000,000
Preferred stock, par or stated value per share		$ 0.0001
Common stock, shares authorized		175,000,000
Shares authorized		200,000,000
Subsequent Event [Member] | Oxford Finance LLC [Member]
Subsequent Event [Line Items]
Debt instrument interest terms	The Term Loan bears interest at a floating per annum rate equal to the greater of (i) 8.0% and (ii) the sum of (a) thirty-day U.S. DOLLAR LIBOR rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, plus (b) 7.84%.
Debt instrument maturity date	Nov. 1, 2025
Debt instruments payment terms	The Company is required to make monthly interest only payments under the Oxford Loan on the first calendar day of each month beginning on January 1, 2021.
Debt instrument frequency of payment terms	monthly
Debt instrument, restrictive covenants	The Company’s obligations under the Oxford Loan Agreement will be secured by a security interest in all of its assets, other than its intellectual property. The Company is also subject to certain affirmative and negative covenants.
Debt, final payment fee percentage	5.00%
Subsequent Event [Member] | Term Loan A [Member] | Oxford Finance LLC [Member]
Subsequent Event [Line Items]
Debt instrument loan amount	$ 20,000					$ 20,000
Proceeds from issuance of debt	20,000
Subsequent Event [Member] | Maximum [Member] | Term Loan B [Member] | Oxford Finance LLC [Member]
Subsequent Event [Line Items]
Debt instrument loan amount	$ 5,000
Subsequent Event [Member] | Minimum [Member] | Oxford Finance LLC [Member]
Subsequent Event [Line Items]
Debt instrument interest rate	8.00%
Debt instrument, basis spread on variable rate	7.84%
Debt instrument, description of variable rate basis	(a) thirty-day U.S. DOLLAR LIBOR rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, plus (b) 7.84%
Subsequent Event [Member] | IPO [Member]
Subsequent Event [Line Items]
Stock issued during the period shares new issues shares		7,500,000
Sale of stock issue price per share		$ 16
Sale of stock consideration received on the transaction		$ 108,000
Warrants granted to purchase common stock		7,608
Subsequent Event [Member] | Over-Allotment Option [Member]
Subsequent Event [Line Items]
Stock issued during the period shares new issues shares	951,837
Sale of stock issue price per share	$ 16
Sale of stock consideration received on the transaction	$ 14,200
Subsequent Event [Member] | 2020 Incentive Plan [Member]
Subsequent Event [Line Items]
Annual increase of shares			4.00%
Subsequent Event [Member] | 2020 Incentive Plan [Member] | Initial Amount [Member]
Subsequent Event [Line Items]
Number of shares authorized			2,200,000
Subsequent Event [Member] | 2020 Incentive Plan [Member] | Initial Aggregate From 2016 And 2020 Plans [Member] | Maximum [Member]
Subsequent Event [Line Items]
Number of shares authorized			5,078,295
Subsequent Event [Member] | 2020 Employee Stock Purchase Plan [Member]
Subsequent Event [Line Items]
Annual increase of shares			1.00%
Number of shares authorized			360,000
Subsequent Event [Member] | 2020 Employee Stock Purchase Plan [Member] | Maximum [Member]
Subsequent Event [Line Items]
Number of shares authorized			3,220,520