Foghorn Therapeutics (FHTX) 10-K 18 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Jan. 31, 2021	Oct. 23, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Registrant Name	Foghorn Therapeutics Inc.
Entity Central Index Key	0001822462
Current Fiscal Year End Date	--12-31
Document Annual 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	Yes
Entity Interactive Data Current	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Public Float			$ 291.7
Entity Shell Company	false
Trading Symbol	FHTX
Title of 12(b) Security	Common Stock, $0.0001 Par Value
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		36,816,714
ICFR Auditor Attestation Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 92,795	$ 14,981
Marketable securities	92,975	0
Restricted cash		541
Prepaid expenses and other current assets	4,917	1,363
Total current assets	190,687	16,885
Property and equipment, net	19,528	2,683
Restricted cash	1,733	1,733
Other assets	842	11
Operating lease right-of-use assets	42,804	1,030
Total assets	255,594	22,342
Current liabilities:
Accounts payable	3,680	3,439
Accrued expenses	9,161	3,701
Operating lease liabilities	3,981	1,360
Notes payable, net of discount		4,152
Deferred revenue	2,024
Total current liabilities	18,846	12,652
Notes payable, net of discount and current portion	19,654	10,960
Operating lease liabilities, net of current portion	58,361	157
Deferred revenue, net of current portion	12,546
Preferred stock warrant liability		45
Total liabilities	109,407	23,814
Commitments and contingencies (Note 13)
Convertible preferred stock (Series A-1, A-2 and B), $0.0001 par value; no shares and 28,629,622 shares authorized at December 31, 2020 and 2019, respectively; no shares and 28,615,546 shares issued and outstanding at December 31, 2020 and 2019, respectively		86,544
Stockholders' deficit:
Preferred stock, $0.0001 par value; 25,000,000 shares authorized as of December 31, 2020; no shares issued or outstanding
Common stock, $0.0001 par value; 175,000,000 and 46,600,000 shares authorized at December 31, 2020 and 2019, respectively; 36,790,946 shares issued and outstanding at December 31, 2020 and 5,762,745 shares issued and 4,870,851 shares outstanding at December 31, 2019	4
Additional paid-in capital	309,126	6,120
Accumulated other comprehensive loss	(7)
Accumulated deficit	(162,936)	(94,136)
Total stockholders' equity (deficit)	146,187	(88,016)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 255,594	$ 22,342

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Common Stock, Par Value Per Share	$ 0.0001	$ 0.0001
Common Stock, Shares Authorized	175,000,000	46,600,000
Common Stock, Shares, Issued	36,790,946	5,762,745
Common Stock, Shares, Outstanding	36,790,946	4,870,851
Preferred Stock, Par Value Per Share	$ 0.0001
Preferred Stock Authorized	25,000,000	28,629,622
Preferred Stock, Shares, Issued	0	28,615,546
Proferred Stock, Shares, Outstanding	0	28,615,546
Convertible Preferred Stock [Member]
Convertible Preferred Stock, Par Value Per Share	$ 0.0001	$ 0.0001
Convertible Preferred Stock, Shares Authorized	0	28,629,622
Convertible Preferred Stock, Shares Issued	0	28,615,546
Convertible Preferred Stock, Shares Outstanding	0	28,615,546

Consolidated Statements Of Oper

Consolidated Statements Of Operations And Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 430
Operating expenses:
Research and development	57,715	$ 44,362
General and administrative	11,246	6,722
Total operating expenses	68,961	51,084
Loss from operations	(68,531)	(51,084)
Other income (expense):
Interest expense	(979)	(540)
Interest income and other income (expense), net	1,001	495
Change in fair value of preferred stock warrant liability	(69)	1
Condensed Consolidated Statements Of Operations And Comprehensive Loss	(222)
Total other expense, net	(269)	(44)
Net loss	$ (68,800)	$ (51,128)
Net loss per share attributable to common stockholders—basic and diluted	$ (6.23)	$ (12.20)
Weighted average common shares outstanding—basic and diluted	11,046,802	4,191,793
Comprehensive loss:
Net loss	$ (68,800)	$ (51,128)
Other comprehensive loss:
Unrealized losses on marketable securities	(7)
Total other comprehensive loss	(7)
Total comprehensive loss	$ (68,807)	$ (51,128)

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO [Member]	Common Stock [Member]	Common Stock [Member]IPO [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]IPO [Member]	Accumulated Other Comprehensive Loss [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
Beginning balance at Dec. 31, 2018	$ (39,273)				$ 3,735			$ (43,008)
Beginning balance (Shares) at Dec. 31, 2018			3,475,152
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
Issuance of common stock upon exercise of stock options	691				691
Issuance of common stock upon exercise of stock options , shares			503,808
Vesting of restricted stock, shares			891,891
Stock-based compensation expense	1,694				1,694
Net loss	(51,128)							(51,128)
Ending balance at Dec. 31, 2019									$ 86,544
Ending balance (Shares) at Dec. 31, 2019									28,615,546
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
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
Conversion of preferred stock to common stock									$ (176,405)
Conversion of preferred stock to common stock	$ 176,405		$ 3		176,402
Conversion of preferred stock to common stock, shares									(40,623,413)
Conversion of preferred stock to common stock, shares			21,958,588
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs		$ 122,121		$ 1		$ 122,120
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares				8,451,837
Conversion of warrant liability to equity upon closing of initial public offering	114				114
Issuance of common stock in connection with the cashless exercise of warrants			6,728
Issuance of common stock upon exercise of stock options	$ 1,221				1,221
Issuance of common stock upon exercise of stock options , shares	611,048		611,048
Issuance of warrants in connection with notes payable	$ 188				188
Vesting of restricted stock, shares			891,894
Stock-based compensation expense	2,961				2,961
Unrealized loss on marketable securities	(7)						$ (7)
Net loss	(68,800)							(68,800)
Ending balance (Shares) at Dec. 31, 2020									0
Ending balance at Dec. 31, 2020	$ 146,187		$ 4		$ 309,126		$ (7)	$ (162,936)
Ending balance (Shares) at Dec. 31, 2020	36,790,946		36,790,946

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Series B Convertible Preferred Stock [Member]
Stock Issuance Costs	$ 198	$ 6

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (68,800)	$ (51,128)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,961	1,694
Depreciation and amortization expense	1,327	693
(Gain) loss on disposal of property and equipment	(206)	11
Loss on debt extinguishment	222
Change in fair value of preferred stock warrant liability	69	(1)
Noncash lease expense	4,201	1,100
Noncash interest expense	238	99
Accretion of discount on marketable securities	(1)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(4,670)	(991)
Accounts payable	474	1,211
Accrued expenses and other current liabilities	3,194	2,137
Operating lease liabilities	15,135	(1,160)
Deferred revenue	14,570
Net cash used in operating activities	(31,286)	(46,335)
Cash flows from investing activities:
Purchases of property and equipment	(16,183)	(968)
Proceeds from sale of property and equipment	250	4
Purchases of marketable securities	(92,981)
Net cash used in investing activities	(108,914)	(964)
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts, commissions and offering costs	122,121
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	1,221	691
Proceeds from issuance of notes payable, net of issuance costs	19,800	7,984
Repayment of notes payable	(15,530)
Net cash provided by financing activities	217,473	23,969
Net increase (decrease) in cash, cash equivalents and restricted cash	77,273	(23,330)
Cash, cash equivalents and restricted cash at beginning of period	17,255	40,585
Cash, cash equivalents and restricted cash at end of period	94,528	17,255
Supplemental cash flow information:
Cash paid for interest	660	420
Supplemental disclosure of noncash investing and financing information:
Purchases of property and equipment included in accounts payable and accrued expenses	2,400	$ 367
Issuance of warrants in connection with notes payable	188
Conversion of Redeemable Convertible stock [Member]
Supplemental disclosure of noncash investing and financing information:
Conversion of redeemable convertible preferred stock to common stock	176,405
Conversion of Warrant Liability to Equity [Member]
Supplemental disclosure of noncash investing and financing information:
Conversion of redeemable convertible preferred stock to common stock	114
Conversion of Notes Payable in to Warrants [Member]
Supplemental disclosure of noncash investing and financing information:
Issuance of warrants in connection with notes payable	$ 188

Consolidated Statements of Ca_2

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 92,795	$ 14,981
Restricted cash (current and non-current)	1,733	2,274
Cash, cash equivalents and restricted cash at end of period	$ 94,528	$ 17,255

Nature of Business and Basis of

Nature of Business and Basis of Presentation	12 Months Ended
	Dec. 31, 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 clinical-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 as a Delaware corporation. The Company is headquartered in Cambridge, Massachusetts. The Company is subject to risks similar to those of other early-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 On October , , the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold shares of its common stock at a public offering price of $ per share, resulting in net proceeds of $ million, after deducting underwriting discounts and commissions and other offering expenses. On November , , the Company issued and sold an additional shares of common stock at the IPO price of $ per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $ 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 and the Company’s outstanding warrants to purchase Series A Preferred Stock automatically became warrants to purchase an aggregate of shares of common stock (see Note 7). 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. (see Note 10), 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 $68.8 million and $51.1 million for the years ended December 31, 2020 and 2019, respectively. As of December 31, 2020, the Company had an accumulated deficit of $162.9 million. The Company expects to continue to generate operating losses in the foreseeable future. As of the issuance date of these consolidated financial statements the Company expects that its cash , and marketable securities 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 consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying 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”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 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, cash equivalents and marketable securities. As of December 31, 2020, the Company maintained cash, cash equivalents and marketable securities at financial institutions 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 programs. These programs could be adversely affected by a significant interruption in these services or the availability of materials. Cash equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Marketable Securities The Company’s marketable securities are classified as available-for-sale and are carried at fair value with the unrealized gains and losses reported as a component of accumulated other comprehensive loss in stockholders’ equity (deficit). Realized gains and losses and declines in value judged to be other than temporary are included as a component of interest income and other income (expense), net based on the specific identification method. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations. Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on the Company’s leased facilities and credit cards. These amounts are classified as restricted cash (current and non-current) 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 depreciated once placed into service. 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. Impairment of long-lived assets The Company evaluates its long-lived assets, which consist primarily of property and equipment and operating lease right-of-use 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 marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). 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 2 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. Classification of convertible preferred stock The Company classified its convertible preferred stock outstanding prior to the IPO outside of stockholders’ equity (deficit) on the consolidated balance sheet because the holders of such shares had redemption rights in the event of a deemed liquidation that, in certain situations, is not solely within the control of the Company. Research and development costs Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, and external costs of vendors engaged to conduct research, preclinical and clinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development over the period to which they relate. Costs for research and development activities are expensed in the period in which they are incurred. Payments for such activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense. Determining the prepaid and accrued balances at the end of any reporting period incorporate certain judgments and estimates by management that are based on information available to the Company including information provided by vendors regarding the progress to completion of specific tasks or costs incurred. Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Leases In accordance with ASC 842, Leases, right-of-use In addition to rent, the leases may require the Company to pay additional costs, such as utilities, maintenance and other operating costs, which are generally referred to as non-lease non-lease non-lease right-of-use Stock-based compensation The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees non-employees, The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Net loss per share In October 2020, upon the closing of the IPO, all outstanding shares of the Company’s convertible preferred stock automatically converted into 21,958,588 shares of the Company’s common stock. Prior to this conversion, the Company followed 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 entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reported a net loss, such losses were not allocated to such participating securities. In periods in which the Company reported a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 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: December 31, 2020 2019 Stock options to purchase common stock 5,016,460 3,886,489 Warrants to purchase common stock 18,445 — Convertible preferred stock (as converted to common stock) — 15,467,863 Unvested restricted common stock — 891,894 Warrants to purchase convertible preferred stock (as converted to common stock) — 7,608 5,034,905 20,253,854 Segments Operating segments are defined as components of an entity for which separate discrete financial information is made available and that is regularly evaluated by the chief operating decision maker (“CODM”) in making decisions regarding resource allocation and assessing performance. The Company’s CODM is its chief executive officer and the Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is focused on pioneering the discovery and development of a new class of medicines targeting genetically determined dependencies within the chromatin regulatory system. Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2020, the Company’s only element of other comprehensive loss was unrealized losses on available for sale debt securities. For the year ended December 31, 2019, there was no difference between net loss and comprehensive loss. Income taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to the provision for income taxes. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step more-likely-than-not 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. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326) No. 2019-10,

Marketable Securities and Fair

Marketable Securities and Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Marketable Securities and Fair Value Measurements	3. Marketable Securities and Fair Value Measurements As of December 31, 2020, available for sale marketable securities by security type consisted of (in thousands): Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value U . S . $ 92,982 $ — $ (7 ) $ 92,975 Total $ 92,982 $ — $ (7 ) $ 92,975 The Company had no marketable securities as of December 31, 2019. The following tables present the Company’s fair value hierarchy for its assets and liabilities, which are measured at fair value on a recurring basis (in thousands): Fair Value Measurements at Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 48,770 $ — $ — $ 48,770 U .S . — 42,997 — 42,997 Marketable securities: U . . — 92,975 — 92,975 Total $ 48,770 $ 135,972 $ — $ 184,742 Fair Value Measurements at Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 14,951 $ — $ — $ 14,951 Liabilities: Preferred stock warrant liability $ — $ — $ 45 $ 45 The preferred stock warrant liability as of December 31, 2019 consisted of the fair value of warrants to purchase 14,076 shares of Series A-1 per share and was based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy. The Company’s valuation of the preferred stock warrants utilized the Black-Scholes option-pricing model, which incorporated assumptions and estimates to value the preferred stock warrants. The Company assessed these assumptions and estimates at the end of each reporting period. Changes in the fair value of the preferred stock warrants were recognized within other income (expense) in the consolidated statements of operations and comprehensive loss and were not material during the years ended December 31, 2020 and 2019. In October 2020, upon closing of the IPO, the warrants to purchase 14,076 shares of Series A-1 shares of common stock. The Company remeasured the warrants as of the IPO date and reclassified the carrying value of the warrants from a non-current liability to additional paid-in capital in its consolidated balance sheet. During the year ended December 31, 2020, these warrants were exercised in full.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Laboratory equipment $ 3,740 $ 3,202 Furniture and fixtures 815 337 Computer equipment and software 100 81 Leasehold improvements 16,961 75 Assets not yet placed in service 15 280 21,631 3,975 Less: Accumulated depreciation and amortization (2,103 ) (1,292 ) $ 19,528 $ 2,683 Depreciation and amortization expense was $1.3 million and $0.7 million for the years ended December 31, 2020 and 2019, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following (in thousands): December 31, 2020 2019 Accrued employee compensation and benefits $ 3,513 $ 1,867 Accrued construction in progress 2,385 119 Accrued external research and development expenses 2,146 1,384 Accrued professional fee s 979 274 Other 138 57 $ 9,161 $ 3,701

Notes Payable

Notes Payable	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Notes Payable	6. Notes Payable Long-term debt consisted of the following (in thousands): December 31, 2020 2019 Principal amount of long-term debt $ 20,000 $ 15,000 Less: Current portion of long-term debt — (4,152 ) Long-term debt, net of current portion 20,000 10,848 Final payment fee 1,000 530 Debt discount, net of accretion (1,346 ) (418 ) Long-term debt, net of discount and current portion $ 19,654 $ 10,960 Loan and Security Agreements As of December 31, 2019, the Company had outstanding loans under its amended loan and security agreement with Comerica Bank (the “Loan”) of A final payment fee of 3.0% of the aggregate amounts drawn under Term Loan A and 4.0% under Term Loan B was 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 was 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. In November 2020, the Company used a portion of the proceeds from its new loan and security agreement described below to repay the outstanding principal balance under Term Loan A and Term Loan B plus unpaid accrued interest and the final payment fee. The Company recorded a loss 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 an aggregate principal amount of $20.0 million (Oxford Term Loan A) and up to an additional $5.0 million (Oxford Term Loan B). On November 19, 2020, the Company borrowed $20.0 million under the Oxford Term Loan A. (a) thirty-day The Company is required to make monthly interest only payments under the Oxford Loan 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 “Maturity Date”). At the Company’s option, the Company may elect to prepay the loans subject to a prepayment fee equal to the following percentage of the principal amount being prepaid: 2% if an advance is prepaid during the first 12 months following the applicable advance date, 1% if an advance is prepaid after 12 months but prior to 24 months following the applicable advance date, and 0.5% if an advance is prepaid any time after 24 months following the applicable advance date but prior to the Maturity Date. The Company’s obligations under the Oxford Loan Agreement are 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 including In addition, in paid-in debt which is being amortized to interest expense over the term of the Oxford Loan of five years. In addition, the Company incurred debt issuance costs of $0.2 million. As of December 31, 2020, the interest rate applicable to outstanding borrowings under the Oxford Loan was As of December 31, 2020, future principal payments due are as follows (in thousands): 2021 $ — 2022 — 2023 833 2024 10,000 2025 9,167 $ 20,000

Convertible Preferred Stock and

Convertible Preferred Stock and Preferred Stock Warrants	12 Months Ended
	Dec. 31, 2020
Stockholders' Equity Note [Abstract]
Convertible Preferred Stock and Preferred Stock Warrants	7. Convertible Preferred Stock and Preferred Stock Warrants Convertible Preferred Stock The Company 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 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. 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. As of December 31, 2019, Preferred Stock Preferred Stock Carrying Value Liquidation Common Stock Series A-1 9,707,826 9,693,750 $ 9,609 $ 9,694 5,239,865 Series A-2 10,804,165 10,804,165 16,179 16,206 5,840,089 Series B Preferred Stock 8,117,631 8,117,631 60,756 60,882 4,387,909 28,629,622 28,615,546 $ 86,544 $ 86,782 15,467,863 Upon the closing of the IPO in October 2020, the Company’s Preferred Stock automatically converted into 21,958,588 shares of common stock. Warrants to Purchase Preferred Stock I A-1

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Common Stock	8. Common Stock On October 27, 2020, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold 7,500,000 shares of its common stock at a public offering price of $16.00 per share, resulting in net proceeds of $107.9 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 7). Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation 2016 Stock incentive plan The Company’s 2016 Stock Incentive Plan, (the “2016 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options and other equity awards to employees, directors and consultants of the Company. Upon the effectiveness of the 2020 Equity Incentive Plan (the “2020 Plan”) in October 2020, the Company ceased granting additional awards under the 2016 Plan. 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 awards, restricted stock units and other stock-based awards . The number of shares initially reserved for issuance under the 2020 Plan wa s (i) shares (the “share pool”), plus (ii) the number of shares of common stock available for issuance under the 2016 Plan as of the effective date of the 2020 Plan, plus the number of shares of common stock underlying awards under the 2016 Plan that on or after the date of adoption expire or become unexercisable without delivery of shares, are forfeited to, or repurchased for cash, are settled in cash, or otherwise become available again for grant under the 2016 Plan, in each case, in accordance with its terms (up to an aggregate of shares). As of December 31, 2020, shares remained available for future grants under the 2020 Plan. The share pool will automatically increase on January 1 of each year from 2021 to 2030 by the lesser of (i) four percent of the number of shares of our common stock outstanding as of the close of business on the immediately preceding December 31 and (ii) the number of shares determined by the board of directors on or prior to such date for such year. The number of shares reserved for issuance under the 2020 Plan was increased by 1,471,576 shares effective January 1, 2021. The 2020 Plan and 2016 Plan are administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted with service-based vesting conditions generally vest over years and expire after years. The exercise price for stock options granted is not less than the fair value of common stock on the date of grant. The Company bases fair value of common stock on the quoted market price. Prior to the IPO, the board of directors determined the value the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional relevant factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant. 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. The aggregate number of shares of common stock available for purchase pursuant to the exercise of options under the ESPP is 360,000 shares, plus an automatic annual increase, as of January 1 of each year from 2021 to 2030, equal to the lesser of (i) one percent of the number of shares of common stock outstanding as of the close of business on the immediately preceding December 31 and (ii) the number of shares determined by the board of directors on or prior to such date for such year (up to a maximum of 3,220,520 shares). The number of shares reserved for issuance under the ESPP was increased by 367,894 shares effective January 1, 2021. As of December 31, 2020, no offering periods have commenced under the ESPP. Stock option valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. Prior to the IPO, the Company was a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted in 2020 and 2019: Year Ended December 31, 2020 2019 Risk-free interest rate 0.4 % 2.2 % Expected volatility 78.5 % 78.2 % Expected dividend yield — — Expected term (in years) 6.1 6.0 The following table summarizes the Company’s option activity during the year ended December 31, 2020: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2019 3,886,489 $ 2.39 Granted 1,922,377 8.87 Exercised (611,048 ) 1.99 Forfeited (181,358 ) 2.10 Outstanding as of December 31, 2020 5,016,460 $ 4.93 8.5 $ 76,941 Vested and expected to vest as of 5,016,460 $ 4.93 8.5 $ 76,941 Options exercisable as of December 31, 2020 1,508,122 $ 1.91 7.4 $ 27,683 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2020 and 2019 was $3.5 million and $1.2 million, respectively. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2020 and 2019 was $6.01 per share and $2.54 per share, respectively. Restricted common stock During 2015, the Company issued and sold 4,459,459 shares of restricted common stock at par value to the scientific founders of the Company. The shares were subject to vesting over a period of five years and began vesting upon the closing of the Series A-1 and were fully vested as of December 31, 2020. The following table summarizes the Company’s restricted common stock activity during the year ended December 31, 2020: Shares Unvested restricted common stock as of December 31, 2019 891,894 Issued — Vested (891,894 ) Unvested restricted common stock as of December 31, 2020 — The aggregate fair value of restricted stock that vested during the years ended December 2020 and 2019 was $4.8 million and $3.3 million, respectively. Stock-based compensation The Company recorded stock-based compensation expense related to common stock options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2020 2019 Research and development expenses $ 1,724 $ 1,140 General and administrative expenses 1,237 554 $2,961 $1,694 As of December 31, 2020, total unrecognized compensation cost was $13.5 million, which is expected to be recognized over a weighted average period of 3.6 years.

Collaboration Agreement

Collaboration Agreement	12 Months Ended
	Dec. 31, 2020
Disclosure Text Block [Abstract]
Collaboration Agreement	10. 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 $15.0 million from Merck, and is eligible to receive up to $245.0 million upon achievement of specified research, development and regulatory milestones by any product candidate generated by the collaboration, and up to $165.0 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 mid-single 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, manufacture 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 was 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. The upfront payment of using the cost-to-cost method, which Under the cost-to-cost method, the revenue catch-up As of December 31, 2020, the aggregate amount of the transaction price related to the unsatisfied portion of the performance obligation is $14.6 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 December 31, 2020, the Company had recorded $0.4 million of revenue under the Collaboration Agreement.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes During the years ended December 31, 20 20 income tax benefits for the net deferred tax assets comprised primarily of net operating losses incurred and research and development tax credits generated in each year, due to its uncertainty of realizing a benefit from these items. All of the Company’s operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 20 20 2019 Federal statutory income tax rate (21.0 )% (21.0 )% State taxes, net of federal benefit (6.4 ) (7.1 ) Federal and state research and development tax credits (3.7 ) (2.2 ) Other (0.5 ) 0.4 Change in deferred tax asset valuation allowance 31.6 29.9 Effective income tax rate 0.0 % 0.0 % Net deferred tax assets consisted of the following (in thousands): December 31, 20 20 2019 Deferred tax assets: Net operating loss carryforwards $ 39,289 $ 23,769 Research and development tax credit carryforwards 4,955 2,415 Capitalized start-up 174 190 Accrued expenses 1,010 522 Stock-based compensation 436 74 Operating lease liabilities 17,032 414 Total deferred tax assets 62,896 27,384 Deferred tax liabilities: Depreciation (2,819 ) (482 ) Operating lease right-of-use (11,694 ) (281 ) Total deferred tax liabilities (14,513 ) (763 ) Valuation allowance (48,383 ) (26,621 ) Net deferred tax assets $ — $ — As of December 31, 20 20 20 Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Sections 382 and 383 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income or tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. Further, until a study is completed by the Company and any limitation is known, no amounts are being recorded as an uncertain tax position. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products that would generate revenue from product sales and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 20 20 The valuation allowance increased by $21.8 million and $15.3 million during the years ended December 31, 20 20 As of December 31, 20 20 recorded any amounts for unrecognized tax benefits. The Company files income tax returns in the U.S. and Massachusetts, as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state, and local income tax authorities for all tax years in which a loss carryforward is available.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	12. Leases 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 resulted in additional rent payments to the landlord over the lease term. During the year ended December 31, 2020, $15.4 million of leasehold improvements were reimbursed by the landlord, which resulted in an increase to operating lease liabilities. The Company is 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. The Company is required to maintain a cash balance of $1.7 million to secure a letter of credit associated with the lease. This amount was classified as restricted cash (non-current) 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-use right-of-use The Company had a lease for office and laboratory facilities in Cambridge, Massachusetts under a noncancelable operating lease that began in August 2017 and expired in March 2025. In April 2020, this lease was assigned and assumed by a related party which became effective in October 2020. The components of lease expense were as follows (in thousands): Year Ended December 31, 2020 2019 Operating lease cost $ 7,032 $ 1,671 Short-term lease cost 61 60 Variable lease cost 1,217 547 $ 8,310 $ 2,278 Supplemental disclosure of cash flow information related to leases was as follows (in thousands): Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 3,086 $ 1,731 Operating lease liabilities arising from obtaining right-of-use $ 38,306 $ 271 Increase in operating lease liabilities and right-of-use $ 7,384 $ 6,513 The weighted-average remaining lease term and discount rate were as follows: 2020 2019 Weighted-average remaining lease term—operating leases (in years) 7.7 1.1 Weighted-average discount rate—operating leases 5.35 % 7.53 % Future annual minimum lease payments under operating leases as of December 31, 20 20 2021 $ 9,814 2022 10,016 2023 10,107 2024 10,347 2025 10,595 Thereafter 30,500 Total future minimum lease payments 81,379 Less: imputed interest (15,104 ) Less: estimated lease incentives (3,933 ) Total operating lease liabilities $ 62,342 Included in the consolidated balance sheet (in thousands): December 31, 2020 Current operating lease liabilities $ 3,981 Operating lease liabilities, net of current portion 58,361 Total operating lease liabilities $ 62,342 Sublease agreement In April 2020, the Company entered into a two-year

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Leases The Company’s commitments under its leases are described in Note 1 2 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	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Defined Contribution Plan	14. 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 was no discretionary match made under the 401(k) Plan as of December 31, 20 20

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Parties	15. 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, to provide general and administrative services to the Company, including certain consulting services and the provision of employee health and dental benefit plans for the Company’s employees. The Company made cash payments for services received under this agreement of $1.2 million and $0.9 million during the years ended December 31, 2020 and 2019, respectively. As of December 31, 2020 and 2019, the Company had no accounts payable to Flagship for costs related to the service agreement. This agreement expired in 2020. 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. In October 2020, this agreement was extended to January 1, 2022, with an option to renew. During

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of presentation	The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying 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”).
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, cash equivalents and marketable securities. As of December 31, 2020, the Company maintained cash, cash equivalents and marketable securities at financial institutions 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 programs. These programs could be adversely affected by a significant interruption in these services or the availability of materials.
Cash equivalents	Cash equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.
Marketable Securities	Marketable Securities The Company’s marketable securities are classified as available-for-sale and are carried at fair value with the unrealized gains and losses reported as a component of accumulated other comprehensive loss in stockholders’ equity (deficit). Realized gains and losses and declines in value judged to be other than temporary are included as a component of interest income and other income (expense), net based on the specific identification method. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations.
Restricted cash	Restricted cash Amounts included in restricted cash represent amounts pledged as collateral for letters of credit required for security deposits on the Company’s leased facilities and credit cards. These amounts are classified as restricted cash (current and non-current)
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 depreciated once placed into service. 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.
Impairment of long-lived assets	Impairment of long-lived assets The Company evaluates its long-lived assets, which consist primarily of property and equipment and operating lease right-of-use
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 marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). 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 2 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.
Classification of convertible preferred stock	Classification of convertible preferred stock The Company classified its convertible preferred stock outstanding prior to the IPO outside of stockholders’ equity (deficit) on the consolidated balance sheet because the holders of such shares had redemption rights in the event of a deemed liquidation that, in certain situations, is not solely within the control of the Company.
Research and development costs	Research and development costs Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock-based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, and external costs of vendors engaged to conduct research, preclinical and clinical development activities as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development over the period to which they relate. Costs for research and development activities are expensed in the period in which they are incurred. Payments for such activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense. Determining the prepaid and accrued balances at the end of any reporting period incorporate certain judgments and estimates by management that are based on information available to the Company including information provided by vendors regarding the progress to completion of specific tasks or costs incurred.
Patent costs	Patent costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Leases	Leases In accordance with ASC 842, Leases, right-of-use In addition to rent, the leases may require the Company to pay additional costs, such as utilities, maintenance and other operating costs, which are generally referred to as non-lease non-lease non-lease right-of-use
Stock-based compensation	Stock-based compensation The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees non-employees, The Company classifies stock-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Net loss per share	Net loss per share In October 2020, upon the closing of the IPO, all outstanding shares of the Company’s convertible preferred stock automatically converted into 21,958,588 shares of the Company’s common stock. Prior to this conversion, the Company followed 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 entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reported a net loss, such losses were not allocated to such participating securities. In periods in which the Company reported a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. The Company reported a net loss attributable to common stockholders for the years ended December 31, 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: December 31, 2020 2019 Stock options to purchase common stock 5,016,460 3,886,489 Warrants to purchase common stock 18,445 — Convertible preferred stock (as converted to common stock) — 15,467,863 Unvested restricted common stock — 891,894 Warrants to purchase convertible preferred stock (as converted to common stock) — 7,608 5,034,905 20,253,854
Segments	Segments Operating segments are defined as components of an entity for which separate discrete financial information is made available and that is regularly evaluated by the chief operating decision maker (“CODM”) in making decisions regarding resource allocation and assessing performance. The Company’s CODM is its chief executive officer and the Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is focused on pioneering the discovery and development of a new class of medicines targeting genetically determined dependencies within the chromatin regulatory system.
Comprehensive loss	Comprehensive loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2020, the Company’s only element of other comprehensive loss was unrealized losses on available for sale debt securities. For the year ended December 31, 2019, there was no difference between net loss and comprehensive loss.
Income taxes	Income taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to the provision for income taxes. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step more-likely-than-not
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. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326) No. 2019-10,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 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: December 31, 2020 2019 Stock options to purchase common stock 5,016,460 3,886,489 Warrants to purchase common stock 18,445 — Convertible preferred stock (as converted to common stock) — 15,467,863 Unvested restricted common stock — 891,894 Warrants to purchase convertible preferred stock (as converted to common stock) — 7,608 5,034,905 20,253,854

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Summary of Available for Sale Marketable Securities By Security Type	As of December 31, 2020, available for sale marketable securities by security type consisted of (in thousands): Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value U . S . $ 92,982 $ — $ (7 ) $ 92,975 Total $ 92,982 $ — $ (7 ) $ 92,975
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following tables present the Company’s fair value hierarchy for its assets and liabilities, which are measured at fair value on a recurring basis (in thousands): Fair Value Measurements at Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 48,770 $ — $ — $ 48,770 U .S . — 42,997 — 42,997 Marketable securities: U . . — 92,975 — 92,975 Total $ 48,770 $ 135,972 $ — $ 184,742 Fair Value Measurements at Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 14,951 $ — $ — $ 14,951 Liabilities: Preferred stock warrant liability $ — $ — $ 45 $ 45

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Sumary of Property and Equipment Net	Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Laboratory equipment $ 3,740 $ 3,202 Furniture and fixtures 815 337 Computer equipment and software 100 81 Leasehold improvements 16,961 75 Assets not yet placed in service 15 280 21,631 3,975 Less: Accumulated depreciation and amortization (2,103 ) (1,292 ) $ 19,528 $ 2,683

Accrued Expenses (Table)

Accrued Expenses (Table)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses	Accrued expenses consisted of the following (in thousands): December 31, 2020 2019 Accrued employee compensation and benefits $ 3,513 $ 1,867 Accrued construction in progress 2,385 119 Accrued external research and development expenses 2,146 1,384 Accrued professional fee s 979 274 Other 138 57 $ 9,161 $ 3,701

Notes Payable (Tables)

Notes Payable (Tables)	12 Months Ended
	Dec. 31, 2020
Long-term Debt, Unclassified [Abstract]
Summary of Long Term Debt	Long-term debt consisted of the following (in thousands): December 31, 2020 2019 Principal amount of long-term debt $ 20,000 $ 15,000 Less: Current portion of long-term debt — (4,152 ) Long-term debt, net of current portion 20,000 10,848 Final payment fee 1,000 530 Debt discount, net of accretion (1,346 ) (418 ) Long-term debt, net of discount and current portion $ 19,654 $ 10,960
Summary of Future Long Term Loan Principal Payments Due	As of December 31, 2020, future principal payments due are as follows (in thousands): 2021 $ — 2022 — 2023 833 2024 10,000 2025 9,167 $ 20,000

Convertible Preferred Stock a_2

Convertible Preferred Stock and Preferred Stock Warrants (Tables)	12 Months Ended
	Dec. 31, 2020
Stockholders' Equity Note [Abstract]
Schedule of stock by class	As of December 31, 2019, Preferred Stock Preferred Stock Carrying Value Liquidation Common Stock Series A-1 9,707,826 9,693,750 $ 9,609 $ 9,694 5,239,865 Series A-2 10,804,165 10,804,165 16,179 16,206 5,840,089 Series B Preferred Stock 8,117,631 8,117,631 60,756 60,882 4,387,909 28,629,622 28,615,546 $ 86,544 $ 86,782 15,467,863

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Weighted-Average Assumptions Used to Estimate Fair Value of Stock Options and Restricted Stock Awards using Black-Scholes Option Pricing Model	The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted in 2020 and 2019: Year Ended December 31, 2020 2019 Risk-free interest rate 0.4 % 2.2 % Expected volatility 78.5 % 78.2 % Expected dividend yield — — Expected term (in years) 6.1 6.0
Schedule of Share Option Activity	The following table summarizes the Company’s option activity during the year ended December 31, 2020: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2019 3,886,489 $ 2.39 Granted 1,922,377 8.87 Exercised (611,048 ) 1.99 Forfeited (181,358 ) 2.10 Outstanding as of December 31, 2020 5,016,460 $ 4.93 8.5 $ 76,941 Vested and expected to vest as of 5,016,460 $ 4.93 8.5 $ 76,941 Options exercisable as of December 31, 2020 1,508,122 $ 1.91 7.4 $ 27,683
Summary of Restricted common stock	Shares Unvested restricted common stock as of December 31, 2019 891,894 Issued — Vested (891,894 ) Unvested restricted common stock as of December 31, 2020 —
Summary of Stock-Based Compensation Expense	The Company recorded stock-based compensation expense related to common stock options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2020 2019 Research and development expenses $ 1,724 $ 1,140 General and administrative expenses 1,237 554 $2,961 $1,694

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Summary of Reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 20 20 2019 Federal statutory income tax rate (21.0 )% (21.0 )% State taxes, net of federal benefit (6.4 ) (7.1 ) Federal and state research and development tax credits (3.7 ) (2.2 ) Other (0.5 ) 0.4 Change in deferred tax asset valuation allowance 31.6 29.9 Effective income tax rate 0.0 % 0.0 %
Summary of Net Deferred Tax Assets	Net deferred tax assets consisted of the following (in thousands): December 31, 20 20 2019 Deferred tax assets: Net operating loss carryforwards $ 39,289 $ 23,769 Research and development tax credit carryforwards 4,955 2,415 Capitalized start-up 174 190 Accrued expenses 1,010 522 Stock-based compensation 436 74 Operating lease liabilities 17,032 414 Total deferred tax assets 62,896 27,384 Deferred tax liabilities: Depreciation (2,819 ) (482 ) Operating lease right-of-use (11,694 ) (281 ) Total deferred tax liabilities (14,513 ) (763 ) Valuation allowance (48,383 ) (26,621 ) Net deferred tax assets $ — $ —

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Summary of Lease Cost	The components of lease expense were as follows (in thousands): Year Ended December 31, 2020 2019 Operating lease cost $ 7,032 $ 1,671 Short-term lease cost 61 60 Variable lease cost 1,217 547 $ 8,310 $ 2,278
Summary of Supplemental Cash Flow Information Related to Leases	Supplemental disclosure of cash flow information related to leases was as follows (in thousands): Year Ended December 31, 2020 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 3,086 $ 1,731 Operating lease liabilities arising from obtaining right-of-use $ 38,306 $ 271 Increase in operating lease liabilities and right-of-use $ 7,384 $ 6,513
Summary of Weighted-Average Remaining Lease Term and Discount Rate Leases	The weighted-average remaining lease term and discount rate were as follows: 2020 2019 Weighted-average remaining lease term—operating leases (in years) 7.7 1.1 Weighted-average discount rate—operating leases 5.35 % 7.53 %
Summary of Minimum Lease Payments	Future annual minimum lease payments under operating leases as of December 31, 2020 were as follows (in thousands): 2021 $ 9,814 2022 10,016 2023 10,107 2024 10,347 2025 10,595 Thereafter 30,500 Total future minimum lease payments 81,379 Less: imputed interest (15,104 ) Less: estimated lease incentives (3,933 ) Total operating lease liabilities $ 62,342
Summary of Operating Lease Liabilities	Included in the consolidated balance sheet (in thousands): December 31, 2020 Current operating lease liabilities $ 3,981 Operating lease liabilities, net of current portion 58,361 Total operating lease liabilities $ 62,342

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	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net income (loss)				$ 68,800	$ 51,128
Retained earnings accumulated deficit				162,936	$ 94,136
Upfront payment received from Merck			$ 15,000	$ 15,000
IPO [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Warrants granted to purchase common stock		7,608
IPO [Member] | Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stock issued during the period shares new issues shares		7,500,000		8,451,837
Sale of stock issue price per share		$ 16
Sale of stock consideration received on the transaction		$ 107,900
Warrants granted to purchase common stock		7,608
Over-Allotment Option [Member] | Common Stock [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	Oct. 31, 2020	Oct. 27, 2020	Dec. 31, 2020	Dec. 31, 2019
Impairment losses on long-lived assets			$ 0	$ 0
IPO [Member]
Conversion of preferred stock, common shares issued	21,958,588	21,958,588

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)	12 Months Ended
	Dec. 31, 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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	5,034,905	20,253,854
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	5,016,460	3,886,489
Warrents to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	18,445
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		15,467,863
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		891,894
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount		7,608

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Summary of Available for Sale Markatable Securties By Security Type (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	$ 92,982
Gross Unrealized Losses	(7)
Estimated Fair Value	92,975
U.S. treasury notes (due within one year) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	92,982
Gross Unrealized Losses	(7)
Estimated Fair Value	$ 92,975

Marketable Securities and Fai_4

Marketable Securities and Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Marketable securities	$ 92,975
Total	184,742
Preferred Stock Warrant Liability [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Preferred stock warrant liability		$ 45
Money Market Funds [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Money market funds	48,770	14,951
US Treasury Securities [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Money market funds	42,997
Marketable securities	92,975
Level 1 [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Total	48,770
Level 1 [Member] | Fair Value, Recurring [Member] | Money Market Funds [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Money market funds	48,770	14,951
Level 2 [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Total	135,972
Level 2 [Member] | Fair Value, Recurring [Member] | US Treasury Securities [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Money market funds	42,997
Marketable securities	$ 92,975
Level 3 [Member] | Fair Value, Recurring [Member] | Preferred Stock Warrant Liability [Member]
FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Preferred stock warrant liability		$ 45

Marketable Securities and Fai_5

Marketable Securities and Fair Value Measurements - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Oct. 27, 2020	Oct. 26, 2020
Marketable Securities	$ 0	$ 92,975
IPO [Member]
Class of warrant or right, number of securities called by warrants or rights			7,608
IPO [Member] | Common Stock [Member]
Class of warrant or right, number of securities called by warrants or rights			7,608
Before IPO [Member] | Series A one Preferred Stock [Member]
Class of warrant or right, number of securities called by warrants or rights				14,076
Preferred Stock Warrant Liability [Member]
Class of warrant or right, number of securities called by warrants or rights	14,076
Warrant issue price	$ 1

Property and Equipment, Net -

Property and Equipment, Net - Sumary of Property and Equipment Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 21,631	$ 3,975
Less: Accumulated depreciation and amortization	(2,103)	(1,292)
Property, Plant and Equipment, Net	19,528	2,683
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,740	3,202
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	815	337
Computer equipment and software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	100	81
Leaseholds and Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	16,961	75
Assets Not Yet Placed In service [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 15	$ 280

Property and Equipment, Net -_2

Property and Equipment, Net - Sumary of Property and Equipment Net (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 1.3	$ 0.7

Accrued Expenses (Detail)

Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 3,513	$ 1,867
Accrued construction in progress	2,385	119
Accrued external research and development expenses	2,146	1,384
Other	138	57
Accrued professional fees	979	274
Total	$ 9,161	$ 3,701

Notes Payable - Summary of Long

Notes Payable - Summary of Long Term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Long-term Debt, Unclassified [Abstract]
Principal amount of long-term debt	$ 20,000	$ 15,000
Less: Current portion of long-term debt		(4,152)
Long-term debt, net of current portion	20,000	10,848
Final payment fee	1,000	530
Debt discount, net of accretion	(1,346)	(418)
Long-term debt, net of discount and current portion	$ 19,654	$ 10,960

Notes Payable - Summary of Futu

Notes Payable - Summary of Future Principal Payments Due (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Long-term Debt, Fiscal Year Maturity [Abstract]
2021	$ 0
2022	0
2023	833
2024	10,000
2025	9,167
Long-term Debt, Gross	$ 20,000	$ 15,000

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Nov. 19, 2020	Nov. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Debt instrument loan amount			$ 20,000	$ 15,000
Final payment fee			1,000	$ 530
Loss on Extinguishment of Debt			$ (222)
Oxford Finance LLC [Member]
Short-term Debt [Line Items]
Interest rate outstanding borrowings loan			8.00%
Debt, weighted average interest rate			6.40%	6.90%
Debt, final payment fee percentage	5.00%
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 LIBOR rate 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 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 are 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
Class of warrant or right, number of securities called by warrants or rights			18,445
Warrants Issue price per share			$ 16.26
Warrants exercisable term			10 years
Warrants not settleable in cash, fair value disclosure	$ 200
Debt instrument terms			At the Company’s option, the Company may elect to prepay the loans subject to a prepayment fee equal to the following percentage of the principal amount being prepaid: 2% if an advance is prepaid during the first 12 months following the applicable advance date, 1% if an advance is prepaid after 12 months but prior to 24 months following the applicable advance date, and 0.5% if an advance is prepaid any time after 24 months following the applicable advance date but prior to the Maturity Date.
Payments of debt issuance costs			$ 200
Term of loan			5 years
Oxford Finance LLC [Member] | Prepayment within First Twelve Months [Member]
Short-term Debt [Line Items]
Percentage of prepayment fee			2.00%
Oxford Finance LLC [Member] | Prepayment After 12 months but prior to 24 months [Member]
Short-term Debt [Line Items]
Percentage of prepayment fee			1.00%
Oxford Finance LLC [Member] | Prepayment After 24 Months [Member]
Short-term Debt [Line Items]
Percentage of prepayment fee			0.50%
Minimum [Member] | Oxford Finance LLC [Member]
Short-term Debt [Line Items]
Debt instrument, description of variable rate basis	LIBOR
Debt instrument, basis spread on variable rate	7.84%
Debt instrument, interest rate, stated percentage	8.00%
Additional Rate In Event Of Default [Member]
Short-term Debt [Line Items]
Debt instrument, interest rate, effective percentage			5.00%
Term Loan A [Member]
Short-term Debt [Line Items]
Debt instrument loan amount				$ 7,000
Debt, final payment fee percentage			3.00%
Term Loan A [Member] | Oxford Finance LLC [Member]
Short-term Debt [Line Items]
Debt instrument loan amount	$ 20,000
Proceeds from issuance of debt	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 loan amount				$ 8,000
Debt, final payment fee percentage			4.00%
Term Loan B [Member] | Base Rate [Member]
Short-term Debt [Line Items]
Debt instrument, interest rate, stated percentage			1.00%
Term Loan B [Member] | Maximum [Member] | Oxford Finance LLC [Member]
Short-term Debt [Line Items]
Debt instrument loan amount	$ 5,000
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%
Term Loan A and B [Member]
Short-term Debt [Line Items]
Final payment fee			$ 500
Loss on Extinguishment of Debt		$ 200

Convertible Preferred Stock a_3

Convertible Preferred Stock and Preferred Stock Warrants - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Oct. 31, 2020	Oct. 27, 2020	Apr. 30, 2020	Aug. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Common Stock [Member]
Issuance of common stock upon exercise of stock options , shares					6,728
IPO [Member]
Conversion of preferred stock, common shares issued	21,958,588	21,958,588
Class of warrant or right, number of securities called by warrants or rights		7,608
IPO [Member] | Common Stock [Member]
Class of warrant or right, number of securities called by warrants or rights		7,608
Issuance of common stock upon exercise of stock options , shares		6,728
Series A-1 Preferred Stock [Member] | Issuance of Notes Payable [Member]
Class of warrant or right, number of securities called by warrants or rights						14,076
Class of Warrant exercise price						$ 1
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			$ 48.1	$ 42

Convertible Preferred Stock a_4

Convertible Preferred Stock and Preferred Stock Warrants - Schedule of stock by class (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Preferred Stock Authorized	25,000,000	28,629,622
Preferred Stock Issued and Outstanding	0	28,615,546
Carrying Value		$ 86,544
Liquidation Preference		$ 86,782
Common Stock Issuable Upon Conversion		15,467,863
Series A-1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock Authorized		9,707,826
Preferred Stock Issued and Outstanding		9,693,750
Carrying Value		$ 9,609
Liquidation Preference		$ 9,694
Common Stock Issuable Upon Conversion		5,239,865
Series A-2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock Authorized		10,804,165
Preferred Stock Issued and Outstanding		10,804,165
Carrying Value		$ 16,179
Liquidation Preference		$ 16,206
Common Stock Issuable Upon Conversion		5,840,089
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock Authorized		8,117,631
Preferred Stock Issued and Outstanding		8,117,631
Carrying Value		$ 60,756
Liquidation Preference		$ 60,882
Common Stock Issuable Upon Conversion		4,387,909

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Nov. 19, 2020	Oct. 27, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Common stock voting rights			Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors.
IPO [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock issued during the period shares new issues shares		7,500,000	8,451,837
Sale of stock issue price per share		$ 16
Sale of stock consideration received on the transaction		$ 107.9
Over-Allotment Option [Member] | Common Stock [Member]
Class of Stock [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	$ 14.2

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jan. 01, 2021	Oct. 21, 2020	Apr. 30, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2015
Share-based Payment Arrangement [Line Items]
Unrecognized compensation costs related to unvested options and unvested restricted stock				$ 13.5
Weighted average period of unrecognized compensation costs				3 years 7 months 6 days
Aggregate Intrinsic Value,Exercised				$ 3.5	$ 1.2
Weighted Average Grant Date Fair value,granted				$ 6.01	$ 2.54
Aggregate fair value of restricted stock that vested				$ 4.8	$ 3.3
Common Stock [Member]
Share-based Payment Arrangement [Line Items]
Sale of restricted common stock to scientific founders						4,459,459
Vesting period			5 years
2020 Incentive Plan [Member]
Share-based Payment Arrangement [Line Items]
Common stock shares available for grant				2,213,094
Number of additional shares authorized	1,471,576
2020 Incentive Plan [Member] | Initial Amount [Member]
Share-based Payment Arrangement [Line Items]
Number of shares authorized		2,200,000
2020 Incentive Plan [Member] | Initial Aggregate From 2016 And 2020 Plans [Member] | Maximum [Member]
Share-based Payment Arrangement [Line Items]
Number of shares authorized		5,078,295
2020 Incentive Plan [Member] | 2021 to 2030 [Member]
Share-based Payment Arrangement [Line Items]
Percentage of automatic annual increase in number of shares		4.00%
2020 Employee Stock Purchase Plan [Member]
Share-based Payment Arrangement [Line Items]
Number of additional shares authorized	367,894
Maximum aggregate number of shares added to plan		3,220,520
2020 Employee Stock Purchase Plan [Member] | Initial Aggregate [Member]
Share-based Payment Arrangement [Line Items]
Number of shares authorized		360,000
2020 Employee Stock Purchase Plan [Member] | 2021 to 2030 [Member]
Share-based Payment Arrangement [Line Items]
Percentage of automatic annual increase in number of shares		1.00%
2020 and 2016 Incentive Plans [Member] | Service Based Vesting [Member]
Share-based Payment Arrangement [Line Items]
Vesting period				4 years
Stock option expiry period				10 years

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Weighted Average Assumptions Used to Estimate Fair Value of Stock Options and Restricted Stock Awards using Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Risk-free interest rate	0.40%	2.20%
Expected volatility	78.50%	78.20%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years 1 month 6 days	6 years

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Share Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance | shares	3,886,489
Number of shares, Granted | shares	1,922,377
Number of shares, Exercised | shares	(611,048)
Number of shares, Forfeited | shares	(181,358)
Ending Balance | shares	5,016,460
Number of shares, Vested | shares	5,016,460
Number of shares, Exercisable | shares	1,508,122
Weighted Average Exercise Price, Beginning balance | $ / shares	$ 2.39
Weighted Average Exercise Price, Granted | $ / shares	8.87
Weighted Average Exercise Price, Exercised | $ / shares	1.99
Weighted Average Exercise Price, Forfeited | $ / shares	2.10
Weighted Average Exercise Price, Ending Balance | $ / shares	4.93
Weighted Average Exercise Price, Vested | $ / shares	4.93
Weighted Average Exercise Price, Exercisable | $ / shares	$ 1.91
Weighted Average Contractual Term, Ending Balance	8 years 6 months
Weighted Average Contractual Term, Vested	8 years 6 months
Weighted Average Contractual Term, Exercisable	7 years 4 months 24 days
Aggregate Intrinsic Value, Ending Balance | $	$ 76,941
Aggregate Intrinsic Value, Vested | $	76,941
Aggregate Intrinsic Value, Exercisable | $	$ 27,683

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Restricted Common Stock (Detail) - Restricted Stock [Member] - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock as of December 31, 2019		891,894
Vested	(891,894)

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-based compensation expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Stock-based compensation expense	$ 2,961	$ 1,694
Research and Development Expenses [Member]
Stock-based compensation expense	1,724	1,140
General and Administrative Expenses [Member]
Stock-based compensation expense	$ 1,237	$ 554

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2020	Dec. 31, 2020
Upfront payment received from Merck	$ 15,000	$ 15,000
Deferred revenue		15,000
Revenue		430
Unsatisfied portion of the performance obligation		14,600
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		$ 400

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Contingency [Line Items]
Description of the limitations on the use of all operating loss carryforwards and tax credit carry forward	Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Sections 382 and 383 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income or tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period.
Increase in valuation allowance	$ 21.8	$ 15.3
Unrecognized tax benefits	0	0
Income Tax Expense (Benefit)	0	$ 0
Domestic Tax Authority [Member]
Income Tax Contingency [Line Items]
Operating loss carry forward	147.3
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Income Tax Contingency [Line Items]
Tax credit carry forward	$ 3.4
Tax Credit Carry forwards Expiration Start Year	2036
Domestic Tax Authority [Member] | Expirable [Member]
Income Tax Contingency [Line Items]
Operating loss carry forward	$ 12.5
Operating Loss Carry forwards Expiration Start Year	2035
Domestic Tax Authority [Member] | Non Expirable [Member]
Income Tax Contingency [Line Items]
Operating loss carry forward	$ 134.8
Operating Loss Carryforward Limit Percentage	80.00%
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating loss carry forward	$ 132.3
Operating Loss Carry forwards Expiration Start Year	2036
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Income Tax Contingency [Line Items]
Tax credit carry forward	$ 2
Tax Credit Carry forwards Expiration Start Year	2031

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)
State taxes, net of federal benefit	(6.40%)	(7.10%)
Federal and state research and development tax credits	(3.70%)	(2.20%)
Other	(0.50%)	0.40%
Change in deferred tax asset valuation allowance	31.60%	29.90%
Effective income tax rate	0.00%	0.00%

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 39,289	$ 23,769
Research and development tax credit carryforwards	4,955	2,415
Capitalized start-up costs	174	190
Accrued expenses	1,010	522
Stock-based compensation	436	74
Operating lease liabilities	17,032	414
Total deferred tax assets	62,896	27,384
Deferred tax liabilities:
Depreciation	(2,819)	(482)
Operating lease right-of-use assets	(11,694)	(281)
Total deferred tax liabilities	(14,513)	(763)
Valuation allowance	(48,383)	(26,621)
Net deferred tax assets	$ 0	$ 0

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	Jun. 29, 2020USD ($)	Apr. 30, 2020ft²	Oct. 31, 2019USD ($)ft²	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2027USD ($)	Jan. 01, 2020USD ($)
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets				$ 42,804	$ 1,030
Increase in operating lease liabilities and right-of-use assets due to lease remeasurement				7,384	6,513
Operating lease liability				62,342
Non Current Restricted Cash				1,733	$ 1,733
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				15,400
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] | Letter of Credit [Member]
Lessee, Lease, Description [Line Items]
Non Current Restricted Cash				1,700
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
Lease Expiration Date	Sep. 30, 2028
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				2,500
Sublease income recorded as other income				$ 1,000

Leases - Summary of Lease Cost

Leases - Summary of Lease Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 7,032	$ 1,671
Short-term lease cost	61	60
Variable lease cost	1,217	547
Total	$ 8,310	$ 2,278

Leases - Summary of Supplementa

Leases - Summary of Supplemental Cash Flow Information Related to Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Cash paid for amounts included in the measurement of operating lease liabilities	$ 3,086	$ 1,731
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	$ 6,513

Leases - Summary of Weighted-A

Leases - Summary of Weighted-Average Remaining Lease Term and Discount Rate Leases (Detail)	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Weighted Average Remaining Lease Term And Discount Rate [Abstract]
Weighted-average remaining lease term—operating leases	7 years 8 months 12 days	1 year 1 month 6 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	Dec. 31, 2020USD ($)
Leases [Abstract]
2021	$ 9,814
2022	10,016
2023	10,107
2024	10,347
2025	10,595
Thereafter	30,500
Total future minimum lease payments	81,379
Less: imputed interest	(15,104)
Less: estimated lease incentives	(3,933)
Total operating lease liabilities	$ 62,342

Leases - Summary of Operating

Leases - Summary of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating Lease, Liability [Abstract]
Current operating lease liabilities	$ 3,981	$ 1,360
Operating lease liabilities, net of current portion	58,361	$ 157
Total operating lease liabilities	$ 62,342

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Jul. 31, 2017	Dec. 31, 2020	Dec. 31, 2019
Shares issued under agreement		36,790,946	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	12 Months Ended
	Oct. 31, 2015	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Accounts payable		$ 3,680	$ 3,439
Flagship Pioneering [Member]
Related Party Transaction [Line Items]
Accounts payable		0	0
Service agreement term	5 years
Payments made to related party under service agreement		$ 1,200	900
Service agreement expiration year		2020
Scientific Founder [Member]
Related Party Transaction [Line Items]
Service agreement term	5 years
Payments made to related party under service agreement		$ 200	$ 200
Service agreement extended date		Jan. 1, 2022
Scientific Founder [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Accounts payable		$ 100