Verve Therapeutics (VERV) 10-Q 10 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 08, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Registrant Name	VERVE THERAPEUTICS, INC.
Entity Central Index Key	0001840574
Trading Symbol	VERV
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	true
Entity Common Stock, Shares Outstanding		48,171,010
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common stock, par value $0.001 per share
Entity File Number	001-40489
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	82-4800132
Entity Address, Address Line One	500 Technology Square, Suite 901
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	603-0070
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 148,134	$ 8,993
Marketable securities	241,076	63,119
Prepaid expenses and other current assets	5,227	1,854
Total current assets	394,437	73,966
Property and equipment, net	6,574	3,984
Restricted cash	5,237	463
Operating lease right-of-use assets	2,335	0
Other long term assets	2,294	0
Total assets	410,877	78,413
Current liabilities:
Accounts payable	3,579	36
Accrued expenses	8,127	7,189
Success payment liability, current portion	6,250	0
Operating lease obligations, current portion	2,295	0
Deferred rent, current portion	0	90
Total current liabilities	20,251	7,315
Operating lease obligations, net of current portion	181	0
Deferred rent, net of current portion	0	125
Success payment liability, net of current portion (See Notes 5, 8 and 14)	5,510	2,806
Antidilution rights liability (See Notes 5, 8 and 14)	0	6,916
Total liabilities	25,942	17,162
Commitments and contingencies (See Note 7 and Note 8)
Stockholders' equity (deficit):
Common stock, $0.001 par value; 200,000,000 and 255,000,000 shares authorized, 48,215,812 and 3,123,424 shares issued at September 30, 2021 and December 31, 2020, respectively; 48,081,404 and 2,585,789 shares outstanding at September 30, 2021 and December 31, 2020, respectively	48	3
Additional paid-in capital	540,402	2,616
Accumulated other comprehensive income	(2)	8
Accumulated deficit	(155,513)	(66,536)
Total stockholders' equity (deficit)	384,935	(63,909)
Total liabilities, convertible preferred stock, and stockholders' equity (deficit)	410,877	78,413
Convertible Preferred Stock
Current liabilities:
Convertible preferred stock (See Note 10)	0	125,160
Stockholders' equity (deficit):
Total stockholders' equity (deficit)		$ 125,160

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Common stock fair value	$ 0.001	$ 0.001
Common Stock, Shares Authorized	200,000,000	255,000,000
Common stock shares issued	48,215,812	3,123,424
Common Stock, Shares, Outstanding	48,081,404	2,585,789

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating expenses:
Research and development	$ 17,495	$ 7,618	$ 42,263	$ 19,795
General and administrative	6,007	1,354	12,264	3,232
Total operating expenses	23,502	8,972	54,527	23,027
Loss from operations	(23,502)	(8,972)	(54,527)	(23,027)
Other income (expense):
Change in fair value of preferred stock tranche liability	0	0	0	2,507
Change in fair value of antidilution rights liability	0	(59)	(25,574)	(1,804)
Change in fair value of success payment liability	700	4	(8,954)	(13)
Interest income and other income (expense), net	53	37	78	164
Total other income (expense), net	753	(18)	(34,450)	854
Net loss	$ (22,749)	$ (8,990)	$ (88,977)	$ (22,173)
Net loss per common share attributable to common stockholders, basic and diluted	$ (0.47)	$ (3.81)	$ (4.52)	$ (10.23)
Weighted-average common shares used in net loss per share attributable to common stockholders, basic and diluted	47,992,773	2,358,924	19,698,450	2,167,842
Comprehensive Loss:
Net loss	$ (22,749)	$ (8,990)	$ (88,977)	$ (22,173)
Other comprehensive (loss) income:
Unrealized (loss) gain on marketable securities	(5)	13	(10)	5
Comprehensive loss attributable to common stockholders	$ (22,754)	$ (8,977)	$ (88,987)	$ (22,168)

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2019	$ (19,553)	$ 25,480	$ 2	$ 1,268	$ 9	$ (20,832)
Beginning Balance (in shares) at Dec. 31, 2019		51,421,404	1,854,438
Issuance of Series A convertible preferred stock, net of issuance costs of $22		$ 36,792
Issuance of Series A convertible preferred stock, net of issuance costs (in shares)		49,749,167
Issuance of common stock to licensor institutions	487			487
Issuance of common stock to licensor institutions (in shares)			187,867
Vesting of restricted common stock (in shares)			134,409
Exercise of stock options	3			3
Exercise of stock options (in shares)			2,025
Unrealized gain (loss) on available-for-sale securities	11				11
Stock-based compensation	95			95
Net loss	(5,603)					(5,603)
Ending Balance at Mar. 31, 2020	(24,560)	$ 62,272	$ 2	1,853	20	(26,435)
Ending balance (in shares) at Mar. 31, 2020		101,170,571	2,178,739
Beginning Balance at Dec. 31, 2019	(19,553)	$ 25,480	$ 2	1,268	9	(20,832)
Beginning Balance (in shares) at Dec. 31, 2019		51,421,404	1,854,438
Conversion of convertible preferred stock to common stock upon closing of initial public offering	0
Issuance of common stock from initial public offering, net of issuance costs of $25,098	(487)
Net loss	(22,173)
Ending Balance at Sep. 30, 2020	(40,822)	$ 125,160	$ 2	2,167	14	(43,005)
Ending balance (in shares) at Sep. 30, 2020		179,519,032	2,447,556
Beginning Balance at Mar. 31, 2020	(24,560)	$ 62,272	$ 2	1,853	20	(26,435)
Beginning Balance (in shares) at Mar. 31, 2020		101,170,571	2,178,739
Issuance of Series A-1 convertible preferred stock, net of issuance costs of $112		$ 62,888
Issuance of Series A-1 convertible preferred stock, net of issuance costs (in shares)		78,348,461
Vesting of restricted common stock (in shares)			134,408
Unrealized gain (loss) on available-for-sale securities	(19)				(19)
Stock-based compensation	100			100
Net loss	(7,580)					(7,580)
Ending Balance at Jun. 30, 2020	(32,059)	$ 125,160	$ 2	1,953	1	(34,015)
Ending balance (in shares) at Jun. 30, 2020		179,519,032	2,313,147
Vesting of restricted common stock	1			1
Vesting of restricted common stock (in shares)			134,409
Unrealized gain (loss) on available-for-sale securities	13				13
Stock-based compensation	213			213
Net loss	(8,990)					(8,990)
Ending Balance at Sep. 30, 2020	(40,822)	$ 125,160	$ 2	2,167	14	(43,005)
Ending balance (in shares) at Sep. 30, 2020		179,519,032	2,447,556
Beginning Balance at Dec. 31, 2020	$ (63,909)	$ 125,160	$ 3	2,616	8	(66,536)
Beginning Balance (in shares) at Dec. 31, 2020	2,585,789	179,519,032	2,585,789
Issuance of Series B convertible preferred stock, net of issuance costs of $241		$ 93,759
Issuance of Series B convertible preferred stock, net of issuance costs (in shares)		77,163,022
Vesting of restricted common stock (in shares)			134,409
Exercise of stock options	$ 72			72
Exercise of stock options (in shares)			48,745
Unrealized gain (loss) on available-for-sale securities	(1)				(1)
Stock-based compensation	670			670
Net loss	(13,263)					(13,263)
Ending Balance at Mar. 31, 2021	(76,431)	$ 218,919	$ 3	3,358	7	(79,799)
Ending balance (in shares) at Mar. 31, 2021		256,682,054	2,768,943
Beginning Balance at Dec. 31, 2020	$ (63,909)	$ 125,160	$ 3	2,616	8	(66,536)
Beginning Balance (in shares) at Dec. 31, 2020	2,585,789	179,519,032	2,585,789
Conversion of convertible preferred stock to common stock upon closing of initial public offering	$ 218,919
Issuance of common stock from initial public offering, net of issuance costs of $25,098	$ (32,490)
Vesting of restricted common stock (in shares)	403,225
Exercise of stock options (in shares)	352,212
Net loss	$ (88,977)
Ending Balance at Sep. 30, 2021	$ 384,935		$ 48	540,402	(2)	(155,513)
Ending balance (in shares) at Sep. 30, 2021	48,081,404		48,081,404
Beginning Balance at Mar. 31, 2021	$ (76,431)	$ 218,919	$ 3	3,358	7	(79,799)
Beginning Balance (in shares) at Mar. 31, 2021		256,682,054	2,768,943
Conversion of convertible preferred stock to common stock upon closing of initial public offering	218,919	$ (218,919)	$ 28	218,891
Conversion of convertible preferred stock to common stock upon closing of initial public offering (in shares)		(256,682,054)	27,720,923
Issuance of common stock from initial public offering, net of issuance costs of $25,098	281,584		$ 16	281,568
Issuance of common stock from initial public offering, net of issuance costs (in shares)			16,141,157
Issuance of common stock to licensor institutions	32,490		$ 1	32,489
Issuance of common stock to licensor institutions (in shares)			878,098
Vesting of restricted common stock (in shares)			134,408
Exercise of stock options	452			452
Exercise of stock options (in shares)			303,467
Unrealized gain (loss) on available-for-sale securities	(4)				(4)
Stock-based compensation	1,351			1,351
Net loss	(52,965)					(52,965)
Ending Balance at Jun. 30, 2021	405,396		$ 48	538,109	3	(132,764)
Ending balance (in shares) at Jun. 30, 2021			47,946,996
Vesting of restricted common stock (in shares)			134,408
Unrealized gain (loss) on available-for-sale securities	(5)				(5)
Stock-based compensation	2,293			2,293
Net loss	(22,749)					(22,749)
Ending Balance at Sep. 30, 2021	$ 384,935		$ 48	$ 540,402	$ (2)	$ (155,513)
Ending balance (in shares) at Sep. 30, 2021	48,081,404		48,081,404

Condensed Consolidated Statem_3

Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020
Statement of Stockholders' Equity [Abstract]
Issuance Costs of Series A Convertible Preferred Stock				$ 22
Issuance Costs of Series A-1 Convertible Preferred Stock			$ 112
Issuance Costs Of Series B Convertible Preferred Stock		$ 241
Issuance Costs Of Common Stock from Initial Public Offering	$ 25,098

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (88,977)	$ (22,173)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,063	1,075
Non-cash lease expense	1,345	0
Amortization of premium on marketable securities	768	201
Stock-based compensation	4,314	408
Change in fair value of preferred stock tranche liabilities	0	(2,507)
Change in fair value of antidilution rights	25,574	1,804
Change in fair value of success payments liabilities	8,954	13
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(5,667)	(1,068)
Accounts payable	3,344	300
Accrued expenses and other liabilities	864	1,582
Operating lease liabilities	(1,419)	0
Deferred rent liability	0	75
Net cash used in operating activities	(49,837)	(20,290)
Cash flows from investing activities:
Purchases of property and equipment	(3,378)	(2,471)
Purchases of marketable securities	(234,490)	(88,324)
Maturities of marketable securities	55,755	27,781
Net cash used in investing activities	(182,113)	(63,014)
Cash flows from financing activities:
Proceeds from issuance of Preferred Stock, net	93,759	92,616
Proceeds from initial public offering, net of underwriting discount	285,214	0
Payment of initial public offering costs	(3,632)	0
Proceeds from exercise of stock options	524	3
Net cash provided by financing activities	375,865	92,619
Increase in cash, cash equivalents and restricted cash	143,915	9,315
Cash, cash equivalents and restricted cash—beginning of period	9,456	3,221
Cash, cash equivalents and restricted cash—end of period	153,371	12,536
Supplemental disclosure of noncash investing and financing activities:
Property and equipment additions included in accounts payable and accrued expenses	361	499
Settlement of tranche right liability	0	7,064
Conversion of convertible preferred stock to common stock upon closing of initial public offering	218,919	0
Settlement of antidilution rights liability by issuing common stock	32,490	487
Right-of-use assets obtained in exchange for new operating lease liabilities	$ 809	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the business and basis of presentation Organization Verve Therapeutics, Inc. (the “Company” or “Verve”) is a genetic medicines company pioneering a new approach to the care of cardiovascular disease, transforming treatment from chronic management to single-course gene editing medicines. The Company was incorporated on March 9, 2018 as Endcadia, Inc., a Delaware corporation, and began operations shortly thereafter. In January 2019, the Company amended its certificate of incorporation to change its name to Verve Therapeutics, Inc. The Company’s principal offices are located in Cambridge, Massachusetts. Liquidity and capital resources Since its inception, the Company has devoted its efforts principally to research and development and raising capital. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry including, but not limited to, technical risks associated with the successful research, development and manufacturing of product candidates, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Current and future programs will require significant research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On June 21, 2021, the Company completed its initial public offering, or IPO, in which the Company issued and sold 16,141,157 shares of its common stock, including 2,105,368 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a public offering price of $ 19.00 per share, for aggregate gross proceeds of $ 306.7 million . The Company received approximately $ 281.6 million in net proceeds after deducting underwriting discounts and estimated offering expenses payable by the Company. In connection with the IPO, all outstanding shares of convertible preferred stock converted into 27,720,923 shares of the Company’s common stock. In connection with the Company's IPO, the Company effected a one-for-9.2595 reverse stock split of the Company’s issued and outstanding common stock. Accordingly, all shares of common stock and per share amounts, as well as the conversion ratio of the Company’s outstanding convertible preferred stock, for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect the reverse stock split, including reclassification of par and additional paid-in capital amounts as a result of the reverse stock split. The accompanying condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. The Company expects that its cash, cash equivalents and marketable securities of $ 389.2 million as of September 30, 2021 will be sufficient to fund its operations and capital expenditure requirements beyond the next 12 months. The Company will need additional financing to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through a combination of equity offerings, debt financings, collaborations, strategic alliances and licensing arrangements. The Company may be unable to raise additional funds or enter into such other agreements when needed on favorable terms or at all. The inability to raise capital as and when needed could have a negative impact on the Company’s financial condition and its ability to pursue its business strategy. The Company will need to generate significant revenue to achieve profitability, and it may never do so. Basis of presentation The accompanying condensed consolidated financial statements have been prepared in accordance with United States generally accepted accounting principles (“GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission ("SEC"). 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”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements, except for the adoption of ASU No. 2016-02, Leases (Topic 842), as discussed further in Note 2, Summary of significant accounting policies and Note 7, Leases. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments that are necessary to present fairly the Company’s financial position as of September 30, 2021, the results of its operations and other comprehensive loss for the three and nine months ended September 30, 2021 and 2020, convertible preferred stock and stockholders’ equity (deficit) for the three and nine months ended September 30, 2021 and 2020 and cash flows for the nine months ended September 30, 2021 and 2020. Such adjustments are of a normal and recurring nature. The results for the three and nine months ended September 30, 2021 are not necessarily indicative of the results for the year ending December 31, 2021 , or for any future period. These interim financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2020, and the notes thereto, included in the Company’s final prospectus for its initial public offering filed pursuant to Rule 424(b)(4) under the Securities Act with the SEC on June 17, 2021.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. S ummary of significant accounting policies The Company's significant accounting policies are disclosed in Note 2, "Summary of significant accounting policies," in the audited consolidated financial statements for the year ended December 31, 2020, and notes thereto, included in the Company’s final prospectus for its IPO filed pursuant to Rule 424(b)(4) under the Securities Act with the SEC on June 17, 2021. Since the date of those financial statements, there have been no changes to its significant accounting policies, except noted below for leases. Cash, cash equivalents and restricted cash Restricted cash represents collateral provided for a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate facilities. A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: September 30, September 30, (in thousands) 2021 2020 Cash and cash equivalents $ 148,134 $ 11,859 Restricted cash 5,237 677 Total cash, cash equivalents and restricted cash $ 153,371 $ 12,536 Recently adopted accounting pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), as subsequently amended, which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors), and replaces the existing guidance in ASC 840. The FASB has issued several updates to the standard which: (i) clarify how to apply certain aspects of the new standard; (ii) provide an additional transition method for adoption of the new standard; (iii) provide a practical expedient for certain lessor accounting; and (iv) amend certain narrow aspects of the guidance. The new standard requires the identification and classification of arrangements that are or contain a lease and requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine the recognition pattern of lease expense over the term of the lease. In addition, a lessee is required to record (i) a right-of-use asset and a lease liability on its balance sheet for all leases with accounting lease terms of more than 12 months regardless of whether it is an operating or finance lease and (ii) lease expense in its consolidated statement of operations for operating leases and amortization and interest expense in its consolidated statement of operations for financing leases. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases under ASC 840. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), which added an optional transition method that allows companies to adopt the standard as of the beginning of the year of adoption as opposed to the earliest comparative period presented. This guidance is effective for the Company for annual periods beginning after December 15, 2021, including interim periods within that fiscal year. Early adoption is permitted. Prior to January 1, 2021, the Company accounted for leases pursuant to ASC 840, Leases. At lease inception, the Company determined if an arrangement was an operating or capital lease. For operating leases, the Company recognized rent expense, inclusive of rent escalations, holidays and lease incentives, on a straight-line basis over the lease term. The difference between rent expense recorded and the amount paid was recorded as deferred rent. The Company presented lease incentives as deferred rent and amortized the incentives as a reduction to rent expense on a straight-line basis over the lease term. The Company classified deferred rent as current and noncurrent liabilities based on the portion of the deferred rent that was scheduled to mature within the next twelve months. The Company adopted Topic 842 during the quarter ended September 30, 2021, with an effective adoption date of January 1, 2021 , using the modified retrospective transition approach which uses the effective date, or January 1, 2021, as the date of initial application. As a result, prior periods are presented in accordance with the previous guidance in ASC 840. At contract inception, the Company determines if an arrangement is or contains a lease. A lease conveys the right to control the use of an identified asset for a period of time in exchange for consideration. If an arrangement is determined to be or contain a lease, the lease is assessed for classification as either an operating or finance lease at the lease commencement date, defined as the date on which the leased asset is made available for use by the Company, based on the economic characteristics of the lease. The Company has elected to apply the package of practical expedients requiring no reassessment of whether any expired or existing contracts are or contain leases, the lease classification of any expired or existing leases, or the capitalization of initial direct costs for any existing leases. As a result of the adoption of ASC 842, the Company recorded (i) an operating lease liability of $ 2.6 million determined using an incremental borrowing rate as of the effective adoption date and (ii) an operating lease right-of-use asset of $ 2.4 million, net of the unamortized balance of prepaid/accrued rent as of the transition date. There was no impact to the Company’s results of operations and cash flows from operations. The Company has elected the short-term lease exemption, which allows the Company to not recognize lease liabilities and right-of-use assets arising from lease arrangements with lease terms of twelve months or less. For each lease with a term greater than twelve months, the Company records a right-of-use asset and lease liability. A right-of-use asset represents the economic benefit conveyed to the Company by the right to use the underlying asset over the lease term. A lease liability represents the Company’s obligation to make lease payments under the arrangement. The Company measures its lease liabilities at lease commencement as the present value of the future lease payments in the contract using the rate implicit in the contract, when available. As an implicit rate has not historically been readily determinable, the Company uses an incremental borrowing rate measured as the rate at which the Company could borrow, on a fully collateralized basis, a commensurate loan in the same currency over a period consistent with the lease term at the commencement date. The incremental borrowing rate is determined based on the Company's synthetic secured rating using the ordered logit model as of the measurement date. The Company measures its right-of-use assets as the lease liability plus initial direct costs and prepaid lease payments, less lease incentives granted by the lessor. Components of a lease are split into three categories: lease components, non-lease components, and non-components. The fixed and in-substance fixed contract consideration (including any consideration related to non-components) are allocated, based on the respective relative fair values, to the lease components and non-lease components. The Company has elected to account for lease and associated non-lease components together as a single lease component for all underlying assets and allocate all of the contract consideration to the lease component only. The Company remeasures right-of-use assets and lease liabilities when a lease is modified, and the modification is not accounted for as a separate contract. A modification is accounted for as a separate contract if the modification grants the Company an additional right of use not included in the original lease arrangement and the increase in lease payments is commensurate with the additional right of use. The Company assesses its right-of-use assets for impairment in a manner consistent with its assessment for long-lived assets held and used in operations. The Company’s operating leases are presented in the condensed consolidated balance sheet as operating lease right-of-use assets, classified as noncurrent assets, and operating lease liabilities, classified as current and noncurrent liabilities. Operating lease expense is recognized on a straight-line basis over the lease term. Variable costs associated with a lease, such as maintenance and utilities, are not included in the measurement of the lease liabilities and right-of-use assets but rather are expensed when the events determining the amount of variable consideration to be paid have occurred.

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	3. Marketable securities Marketable securities by security type consisted of the following: September 30, 2021 (in thousands) Amortized Gross Gross Fair U.S. treasury bills and notes $ 223,434 $ 3 $ ( 5 ) $ 223,432 U.S. agency securities 17,644 1 ( 1 ) 17,644 Total $ 241,078 $ 4 $ ( 6 ) $ 241,076 December 31, 2020 (in thousands) Amortized Gross Gross Fair U.S. treasury bills and notes $ 32,221 $ 3 $ - $ 32,224 U.S. agency securities 30,890 5 - 30,895 Total $ 63,111 $ 8 $ - $ 63,119 The remaining contractual maturities of all marketable securities were less than one year as of September 30, 2021 and December 31, 2020 .

Property and equipment, net

Property and equipment, net	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Property and equipment, net	4. P roperty and equipment, net Property and equipment, net, consist of the following: (in thousands) September 30, 2021 December 31, Lab equipment $ 7,458 $ 3,937 Leasehold improvements 266 259 Furniture and fixtures 566 481 Computer equipment 145 105 Total property and equipment 8,435 4,782 Less accumulated depreciation ( 1,861 ) ( 798 ) Property and equipment, net $ 6,574 $ 3,984 Depreciation expense for the three and nine months ended September 30, 2021 was $ 0.4 million and $ 1.1 million, respectively. Depreciation expense for the three and nine months ended September 30, 2020 was $ 0.6 million and $ 1.1 million, respectively.

Fair value of financial instrum

Fair value of financial instruments	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair value of financial instruments	5. f air value of financial instruments The Company’s financial instruments that are measured at fair value on a recurring basis consist of money market funds, marketable securities, the preferred stock tranche liability as well as certain derivative liabilities (antidilution right liability and success payment liability) pursuant to our license agreements with the President and Fellows of Harvard College, or Harvard, and The Broad Institute, Inc., or the Broad, which license agreements are referred to herein as the Harvard/Broad License Agreement and the Broad License Agreement. The following tables set forth the fair value of the Company’s financial instruments by level within the fair value hierarchy: As of September 30, 2021 (in thousands) Fair Level 1 Level 2 Level 3 Assets Money market funds $ 113,663 $ 113,663 $ - $ - Marketable securities: U.S. treasury bills and notes 223,432 - 223,432 - U.S. agency securities 17,644 - 17,644 - Total assets $ 354,739 $ 113,663 $ 241,076 $ - Liabilities Success payment liability* 11,760 - - 11,760 Total liabilities $ 11,760 $ - $ - $ 11,760 * During the three months ended September 30, 2021, multiple success payments were triggered and $ 6.3 million was due to Harvard and Broad, which was recorded as a current liability at September 30, 2021. As of December 31, 2020 (in thousands) Fair Level 1 Level 2 Level 3 Assets Money market funds $ 6,724 $ 6,724 $ - $ - Marketable securities: U.S. treasury bills and notes 32,224 - 32,224 - U.S. agency securities 30,895 - 30,895 - Total assets $ 69,843 $ 6,724 $ 63,119 $ — Liabilities Success payment liability $ 2,806 $ - $ - $ 2,806 Antidilution rights liability 6,916 - - 6,916 Total liabilities $ 9,722 $ - $ - $ 9,722 Cash Equivalents —Cash equivalents of $ 113.7 million and $ 6.7 million as of September 30, 2021 and December 31, 2020, respectively, consisted of money market funds and are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices in active markets. Marketable Securities— The Company measures its marketable securities at fair value on a recurring basis and classifies those instruments within Level 2 of the fair value hierarchy. Marketable securities are classified within Level 2 of the fair value hierarchy because pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reporting date, and fair value is determined through the use of models or other valuation methodologies. Antidilution Rights Liability —The antidilution rights liability represents the obligation to issue additional shares of common stock to Harvard and Broad following the completion of (1) a defined aggregate level of preferred stock financing and (2) either a sale of the Company’s preferred stock, an initial public offering, or a company sale meeting a certain value threshold. The antidilution rights liability is stated at fair value and is considered Level 3 in the fair value hierarchy because its fair value measurement is based, in part, on significant inputs not observed in the market. The antidilution rights liability related to meeting a defined aggregate level of preferred stock financing was valued using a probability-weighted present value model that considered the probability of meeting the defined aggregate level of preferred stock financing, as well as the fair value of the Company’s common stock. The antidilution rights liability related to the achievement of a specified valuation through either a sale of the Company’s preferred stock, an initial public offering, or a company sale was valued using a Monte Carlo simulation model, which models the value of the liability based on several key variables, including probability of event occurrence, timing of event occurrence, as well as the fair value of the Company’s common stock. In June 2021, upon completion of its IPO, the Company settled the antidilution rights liability in full through the issuance of 878,098 shares of the Company's common stock for a settlement amount of $ 32.5 million. Prior to settlement, the Company remeasured the liability with a corresponding increase of $ 25.6 million to other expense for the nine months ended September 30, 2021 . The Company recorded an increase of $ 0.1 million and $ 1.8 million to other expense for the three and nine months ended September 30, 2020, respectively. Success Payment Liability —The Company is obligated to pay to Harvard and Broad tiered success payments in the event its average market capitalization exceeds specified thresholds for a specified period of time ascending from a high nine-digit dollar amount to $ 10.0 billion, or sale of the Company for consideration in excess of those thresholds. In the event of a change of control or a sale of the Company, the Company is required to pay success payments in cash within a specified period following such event. Otherwise, the success payments may be settled at the Company’s option in either cash or shares of its common stock, or a combination of cash and shares of its common stock. The maximum aggregate success payments that could be payable by the Company is $ 31.3 million (after termination of the Broad License Agreement). The success payments liability is stated at fair value and is considered Level 3 because its fair value measurement is based, in part, on significant inputs not observed in the market. The Company used a Monte Carlo simulation model, which models the value of the liability based on several key variables, including probability of event occurrence, timing of event occurrence, as well as the value of the Company’s common stock. The Company also estimated the likelihood that it would maintain both the Harvard/Broad License Agreement and the Broad License Agreement based on its on-going research efforts. The Company remeasured the liability at fair value with a decrease of $ 0.7 million recorded to other income for the three months ended September 30, 2021 and an increase in of $ 9.0 million recorded to other expense for the nine months ended September 30, 2021 . The Company remeasured the liability at fair value with a decrease of less than $ 0.1 million recorded to other income for the three months ended September 30, 2020 and an increase of less than $ 0.1 million recorded to other expense for the nine months ended September 30, 2020. In September 2021, multiple success payments were triggered and amounts due to Harvard and Broad totaled $ 6.3 million, which was recorded as a current liability as of September 30, 2021. These amounts remained payable at September 30, 2021 and will be settled in cash in the three months ending December 31, 2021. The Company will continue to adjust the remaining success payment liability for changes in fair value until the earlier of the achievement or expiration of the obligation. The primary inputs used in valuing (i) the success payments liability and (ii) the antidilution rights liability associated with the Company’s realization of a certain valuation threshold through either a sale of the Company’s preferred stock, an initial public offering, or a company sale at September 30, 2021 (only applicable to success payments liability) and December 31, 2020, were as follows: At At Fair value of common stock (per share) $ 47.00 $ 8.24 Equity volatility 77 % 105 % Cumulative probability of triggering event n/a 70 % Expected term (in years) n/a 0.50 At December 31, 2020, the fair value of the common stock was determined by management with the assistance of an independent third-party valuation specialist using methods consistent with the AICPA Valuation Guide. The computation of equity volatility was estimated using available information about the historical volatility of stocks of similar publicly traded companies for a period matching the expected term assumption. In addition, the Company incorporated the timing and probability of future events in the calculation of liabilities. The Company also estimated the likelihood that it would maintain both the Harvard/Broad License Agreement and the Broad License Agreement based on its on-going research efforts. The Company applied a 90 % probability of termination of the Broad License Agreement at December 31, 2020. In February 2021, the Company provided written notice to Broad of its election to terminate the Broad License Agreement, which termination became effective in June 2021. The reconciliation of changes in the fair value of financial instruments based on Level 3 inputs for the nine months ended September 30, 2021 is as follows: (in thousands) Antidilution Success Total Balance at December 31, 2020 $ 6,916 $ 2,806 $ 9,722 Issuance of common stock ( 32,490 ) - ( 32,490 ) Changes in fair value 25,574 8,954 34,528 Balance at September 30, 2021 $ - $ 11,760 $ 11,760 The reconciliation of changes in the fair value of financial instruments based on Level 3 inputs for the nine months ended September 30, 2020 is as follows: (in thousands) Preferred Antidilution Success Total Balance at December 31, 2019 $ 9,571 $ 2,044 $ 419 $ 12,034 Issuance of Series A Preferred Stock ( 7,064 ) - - ( 7,064 ) Issuance of common stock - ( 487 ) - ( 487 ) Changes in fair value ( 2,507 ) 1,804 13 ( 690 ) Balance at September 30, 2020 $ - $ 3,361 $ 432 $ 3,793

Accrued expenses

Accrued expenses	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Accrued expenses	6. Accrued expenses Accrued expenses consist of the following: (in thousands) September 30, December 31, Employee compensation and related benefits $ 3,151 $ 1,636 Accrued external research and development expenses 3,904 4,827 Professional fees 648 303 Other 424 423 Total $ 8,127 $ 7,189

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	7. Leases The Company’s operating lease activity is comprised of non-cancelable facility leases for office and laboratory space in Cambridge, Massachusetts. In March 2020, the Company entered into an operating lease agreement for 16,843 square feet of space located at 500 Technology Square, Cambridge, Massachusetts (the “Cambridge Lease”), with the Company acting as subtenant. The term of the Cambridge Lease commenced in August 2020 and is scheduled to expire two years from the commencement date. The lease provides for annual rent escalations. The Company pays for its proportionate share of building operating costs such as maintenance, utilities, and insurance. In June 2020, the Company entered into the First Amendment to the Cambridge Lease adding 2,980 square feet of additional space. In January 2021, the Company entered into the Second Amendment to the Cambridge Lease adding 2,980 square feet of additional space at a commensurate rent increase which was treated as a new contract. In July, 2021, the Company entered into an amendment for its office space at 500 Technology Square and corresponding parking lot to extend the lease term through November 30, 2021 . The renewal operates under the same terms and conditions as the original lease. As a result of the renewal, the right-of-use asset and lease liability were remeasured at the present value of the lease payments not yet paid, discounted by the incremental borrowing rate (IBR) at the remeasurement date. This resulted in a net increase to the lease liability and right-of-use asset of $ 0.8 million. There was no impact on the consolidated statement of operations and comprehensive loss or the consolidated statement of cash flows as a result of the lease renewal. The components of operating lease cost were as follows: (in thousands) Three Months Ended September 30, 2021 Nine Months Ended September 30, 2021 Operating lease costs $ 495 $ 1,418 Variable lease costs 187 530 Total $ 682 $ 1,948 Supplemental cash flow information related to operating leases was as follows: (in thousands) Nine Months Ended September 30, 2021 Cash paid for amounts included in the measurements of lease liabilities: Operating cash flows related to operating leases $ 1,492 Future minimum commitments under non-cancellable leases as of September 30, 2021 were as follows: Years ending December 31, Amount (in thousands) Remainder of 2021 $ 529 2022 2,764 2023 9,668 2024 9,958 2025 10,257 Thereafter 75,605 Total lease payments $ 108,781 Less: interest ( 21 ) Present value of operating lease liabilities $ 108,760 As of September 30, 2021 , the Company’s operating leases were measured using a weighted-average incremental borrowing rate of 1.57 % over a weighted-average remaining lease term of 1.20 years. On August 19, 2021, the Company entered into a lease agreement (the “Lease”) with ARE-MA Region No. 87 Tenant, LLC, a Delaware limited liability company (the “Landlord”), pursuant to which the Company will lease approximately 104,933 square feet of office and laboratory space located at 201 Brookline Avenue, Boston, Massachusetts (the “Premises”). The Premises have not been made available to the Company as of September 30, 2021 and is therefore excluded from the table above, as the Lease has not commenced. The term of the Lease will commence on the earlier of (1) the date the Landlord delivers the Premises to the Company, which the parties expect to be on or prior to August 1, 2022, or (2) the date the Landlord could have delivered the Premises but for any delays attributable to the Company (the “Commencement Date”). The Company’s obligation for the payment of base rent for the Premises begins 90 days after the Commencement Date (the “Rent Commencement Date”) and will initially be $0.8 million per month, which will increase by approximately 3% per annum. The Lease has a term of 10 years, measured from the Rent Commencement Date. The Company has the option to extend the term of the Lease for a period of an additional five years . Under the terms of the Lease, the Landlord has agreed to make up to $ 21.0 million in certain tenant improvements to the Premises to suit the Company’s use (the “Tenant Improvement Allowance”), which amount is included in the base rent set forth in the Lease. In addition to the Tenant Improvement Allowance, the Company may receive up to $ 5.2 million for additional improvements to the Premises, which amount is subject to an 8 % annual interest charge and may be paid in equal monthly installments. In connection with its entry into the Lease and as a security deposit, the Company has provided the Landlord a letter of credit in the amount of approximately $ 4.8 million, which may be reduced to approximately $ 3.5 million on the expiration of the 36-month anniversary of the Rent Commencement Date so long as there are, and have been, no defaults by the Company under the terms of the Lease. The Company also paid a deposit in the amount of $ 0.8 million, which is equal to the first month of base rent. The Landlord has the right to terminate the Lease upon customary events of default. The Company may also termina te the Lease if the Premises are not ready for occupancy within a specified time period after August 1, 2022. The Lease requires the Landlord to perform a scope of work to build-out the base building prior to the construction of the Premises. The Company concluded the accounting commencement date will occur when the Landlord completes the build-out of the base building and control passes to the Company. The Company will assess the classification of the Lease at the accounting commencement date, measure the right-of-use asset and lease liability. Payments made by the Company prior to commencement will be treated as prepaid rent which will increase the right-of-use asset. As of September 30, 2021, the Company has incurred $ 1.4 million related to the build out of the lease space prior to the commencement date. The Company expects to incur approximately $ 11.0 million in additional costs related to tenant improvements. This amount is an estimate and is subject to change as the build out occurs.

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2021
License Agreement [Abstract]
License Agreement	8. License agreements The Company's significant license agreements are disclosed in Note 8, "License Agreements," in the audited consolidated financial statements for the year ended December 31, 2020, and notes thereto, included in the Company’s final prospectus for its IPO filed pursuant to Rule 424(b)(4) under the Securities Act with the SEC on June 17, 2021. Since the date of those financial statements, there have been no changes to its license agreements, except as noted below. Harvard/Broad license agreement and Broad license agreement In March 2019, the Company simultaneously entered into the Harvard/Broad License Agreement and Broad License Agreement (the “license agreements”) for certain base editing technologies pursuant to which the Company received exclusive, worldwide, sublicensable, royalty-bearing licenses under specified patent rights to develop and commercialize licensed products and nonexclusive, worldwide, sublicensable, royalty-bearing licenses under certain patent rights to research and develop licensed products. The Company agreed to use commercially reasonable efforts to develop licensed products in accordance with the development plans, to introduce any licensed products that gain regulatory approval into the commercial market, to market licensed products that have gained regulatory approval following such introduction into the market, and to make licensed products that have gained regulatory approval reasonably available to the public. The term of the agreements will continue until the expiration of the last to expire valid claim. The Company may terminate either of the license agreements without cause upon four months’ prior written notice to Harvard and Broad, unless terminated earlier. In February 2021, the Company provided written notice to Broad of its intent to terminate the Broad License Agreement, which termination was effective in June 2021. As partial consideration for the rights granted under the Harvard/ Broad License Agreement and Broad License Agreement, the Company paid $ 0.3 million in non-refundable upfront license fees and also issued 276,075 shares of its common stock with a fair value of $ 0.3 million. Additional consideration under the license agreements is as follows: Antidilution Rights —The initial shares of common stock issued to Harvard and Broad are subject to antidilution provisions as further described in Note 5, Fair value of financial instruments. The antidilution rights associated with the Company achieving a defined aggregate level of preferred stock financing were partially satisfied in 2019 and fully satisfied in 2020, which settlement amounts totaled $ 0.1 million and $ 0.5 million, respectively, and which amounts were settled through issuances of 121,411 and 187,867 shares of common stock, respectively. The remaining antidilution rights obligation was fully satisfied in the three months ended June 30, 2021 with the Company’s initial public offering. The settlement amount totaled $ 32.5 million, and was settled through issuance of 878,098 shares of common stock. Success Payments —The Company is required to make success payments under the license agreements as further described in Note 5, Fair value of financial instruments. In September 2021, certain success payments were triggered and amounts due to Harvard and Broad totaled $ 6.3 million. These amounts remained payable at September 30, 2021 and will be settled in cash in the three months ending December 31, 2021 . Other Payments —The Company agreed to pay an annual license maintenance fee ranging from low-to-mid five figures to low six figures, depending on the particular calendar year, for each of the license agreements. The Company is responsible for the payment of certain patent prosecution and maintenance costs incurred by Harvard and Broad related to licensed patents. To the extent achieved, the Company is obligated to pay up to an aggregate of $ 46.2 million and $ 108 million in development and sales-based milestones, respectively. If the Company undergoes a change of control during the term of the license agreements, then certain of the milestone payments would be increased by a mid-double-digit percentage. To the extent there are sales of a licensed product, the Company is required to pay low single digit royalties on net sales, for each of the license agreements. The Company is entitled to certain reductions and offsets on these royalties with respect to a licensed product in a given country. Verily agreement The Company elected to terminate its collaboration agreement with Verily Life Sciences LLC ("Verily"), effective June 26, 2020, and has no outstanding amounts due or payable to Verily as of September 30, 2021 . The Company did no t record any amounts as research and development expense in the three and nine months ended September 30, 2021 related to this agreement and recorded $ 0.0 million and $ 1.5 million as research and development expense in the three and nine months ended September 30, 2020, respectively, related to this agreement. Beam license agreement In April 2019, the Company and Beam Therapeutics, Inc. ("Beam") entered into a collaboration and license agreement. As partial consideration for the license rights granted by Beam under this agreement, the Company paid a one-time, nonrefundable fee through issuing 276,075 shares of its common stock with a fair value of $ 0.3 million. To the extent achieved, for each licensed product, the Company is also obligated to pay up to $ 11.3 million in development and regulatory-based milestones and $ 15.0 million in sales-based milestones. To the extent there are sales of a licensed product, the Company is required to pay low-to-mid single digit royalties on net sales. To the extent achieved, for each collaboration product outside of the United States, the Company is obligated to pay up to $ 5.6 million in development and regulatory-based milestones and $ 7.5 million in sales-based milestones. To the extent there are ex-U.S. sales of a collaboration product, the Company is required to pay low-to-mid single digit royalties on net sales. The Company has not triggered any milestone payments pursuant to its arrangement with Beam, and no expense has been recorded for milestones as of September 30, 2021. To the extent there are sales of a delivery technology product, each party will pay the other party low-to-mid single digit royalties based on the annual aggregate worldwide net sales resulting from the sale of each delivery technology product of such paying party; provided, however, that such royalty payments will not apply to net sales of the collaboration products or licensed products. The Company concluded the receipt of any milestone or royalty payments under the agreement was not probable as of September 30, 2021. Beam materials exchange letter agreement In October 2020, the Company and Beam entered into a materials exchange agreement wherein the parties agreed that Beam would provide certain mRNA, gRNA, and protein to the Company and that the Company would provide certain gRNAs to Beam at an agreed upon price per each material provided. For the three and nine months ended September 30, 2021 , the Company did no t purchase any materials from Beam. For the three and nine months ended September 30, 2020 , the Company purchased $ 0.0 million and $ 0.2 million, respectively, as a charge to research and development expense related to materials purchased from Beam. For the three and nine months ended September 30, 2021 , the Company recognized $ 0.0 million and $ 0.2 million, respectively, as a reduction to research and development expense related to reimbursements received for materials sold to Beam. During the three and nine months ended September 30, 2020 , the Company recognized $ 0.2 million and $ 0.3 million, respectively, as a reduction to research and development expense related to reimbursements received for materials sold to Beam. Acuitas agreements Development and option agreement In December 2019, the Company and Acuitas Therapeutics, Inc. ("Acuitas") entered into a development and collaboration agreement, which agreement was amended and restated in October 2020. The Company agreed to reimburse Acuitas on a quarterly basis for its services performed related to the program activities based on an agreed upon number of fulltime employees committed to work on the program at an annual rate per employee, including reimbursement of reasonable external costs. These services commenced during 2020 and the Company recognized research and development expense during the three and nine months ended September 30, 2021 of $ 0.1 million and $ 0.7 million, respectively, related to the reimbursement of research and development services provided by Acuitas and technology maintenance fees and $ 0.6 million and $ 1.3 million for the three and nine months ended September 30, 2020, respectively. In 2020 upon the one-year anniversary of the agreement the Company had exercised one of its options to enter into a non-exclusive license agreement, as further described below. License agreement In October 2020, the Company paid Acuitas a non-refundable, upfront license fee of $ 2.0 million (less a previously paid target reservation fee) to exercise an option with respect to a licensed product and a licensed genome target and entered into a non-exclusive, worldwide license with Acuitas, with a right to sub-license through multiple tiers, under the licensed LNP technology to research, develop and commercialize the licensed products using the LNP technology in connection with the PCSK9 gene target for all human therapeutic or prophylactic uses. To the extent achieved, the Company is also obligated to pay up to an aggregate of $ 9.8 million in clinical and regulatory milestones and $ 9.5 million in sales-based milestones. The milestones have not been achieved and no expense has been recorded for these milestones as of September 30, 2021 .

Preferred Stock Tranche Liabili

Preferred Stock Tranche Liability	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Preferred Stock Tranche Liability	9. Preferred stock tranche liability Included in the terms of the Series A Preferred Stock purchase agreement were certain tranche rights whereupon the Company is obligated to issue, and the Series A Preferred investors have the obligation to purchase 49,749,167 shares of Series A Preferred Stock at $ 0.598 per share upon the Company achieving additional scientific and non-scientific milestones (“third tranche”). In March 2020, the board of directors agreed to waive the final remaining milestones and determined the third tranche milestones, as modified, were achieved. In March 2020, the Company settled the third tranche by issuing 49,749,167 shares of Series A Preferred Stock at a price of $ 0.598 per share for gross proceeds of $ 29.8 million.

Preferred and Common Stock

Preferred and Common Stock	9 Months Ended
	Sep. 30, 2021
Preferred Stock [Abstract]
Preferred and Common Stock	10. Preferred and common stock In January 2021, the Company issued 77,163,022 shares of Series B Preferred Stock at a price of $ 1.2182 per share for gross proceeds of $ 94.0 million. The Company incurred issuance costs in connection with this transaction of $ 0.2 million. In June 2021, the Company amended and restated its certificate of incorporation to authorize 5,000,000 shares of preferred stock, which shares of preferred stock are currently undesignated, and 200,000,000 shares of common stock, $ 0.001 par value per share. In June 2021, the Company completed its IPO, pursuant to which the Company issued and sold 16,141,157 shares of its common stock, including 2,105,368 shares pursuant to the full exercise of the underwriters' option to purchase additional shares, at a public offering price of $ 19.00 per share, for aggregate gross proceeds of $ 306.7 million . The Company received approximately $ 281.6 million in net proceeds, after deducting underwriting discounts and estimated offering expenses payable by the Company. Upon the closing of the IPO, all outstanding shares of the Company's preferred stock automatically converted into 27,720,923 shares of the Company's common stock. In June 2021, the Company issued 878,098 shares of its common stock to Harvard and Broad as final settlement of its antidilution rights obligation. The holders of common stock are entitled to one vote for each share of common stock.

Stock-based compensation

Stock-based compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-based compensation	11. Stock-based compensation The 2018 Equity Incentive Plan, or the 2018 Plan, adopted by the board of directors in August 2018 provided for the grant of qualified incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock and restricted stock units to the Company’s employees, officers, directors, advisors, and outside consultants for the issuance or purchase of shares of the Company’s common stock. The maximum number of shares of common stock that were authorized for issuance under the 2018 Plan was 6,885,653 . In June 2021, the Company's board of directors adopted, and the Company's stockholders approved, the 2021 Stock Incentive Plan, or the 2021 Plan, which became effective on June 16, 2021. The 2021 Plan provides for grant of qualified and nonqualified stock options, stock appreciation rights, restricted and unrestricted stock and stock units, performance awards, and other share-based awards to the Company's employees, directors, advisors and outside consultants. Upon effectiveness of the 2021 Plan, the number of shares of common stock that was reserved for issuance under the 2021 Plan is the sum of: (1) 3,466,530 ; plus (2) the number of shares as is equal to the sum of (x) the number of shares of common stock reserved for issuance under the 2018 Plan that remained available for grant under the 2018 Plan on June 16, 2021 and (y) the number of shares of common stock subject to outstanding awards granted under the 2018 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, commencing on January 1, 2022 and continuing until, and including, January 1, 2031, equal to the lesser of (i) 5 % of the number of shares of common stock outstanding on such date and (ii) the number of shares of common stock determined by our board of directors. As of September 30, 2021 the Company had reserved 4,632,042 shares of the Company's common stock for issuance of stock options and restricted stock, of which 3,980,489 remained outstanding for future grant under the 2021 Plan. Upon effectiveness of the 2021 Plan, the Company ceased granting additional awards under the 2018 Plan. Stock-based compensation expense recorded in the consolidated statements of operations and comprehensive loss is as follows: Three months ended September 30, Nine months ended September 30, (in thousands) 2021 2020 2021 2020 Research and development $ 1,285 $ 130 $ 2,306 $ 250 General and administrative 1,008 83 2,008 158 Total stock-based compensation expense $ 2,293 $ 213 $ 4,314 $ 408 Stock options The following table provides a summary of stock option activity during the nine months ended September 30, 2021: Number of Weighted Weighted Aggregate (2) Outstanding at December 31, 2020 3,888,823 $ 2.13 Granted 2,408,546 13.75 Exercised ( 352,212 ) 1.49 Forfeited ( 39,636 ) 2.56 Outstanding at September 30, 2021 5,905,521 $ 6.92 8.8 $ 237,545 Exercisable at September 30, 2021 1,348,988 $ 1.99 8.1 $ 60,722 Expected to vest after September 30, 2021 (1) 4,556,533 $ 8.38 9.0 $ 176,823 (1) This represents the number of unvested options outstanding as of September 30, 2021 that are expected to vest in the future. (2) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money as of September 30, 2021 . As of September 30, 2021 , there was $ 30.6 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of approximately 3.0 years. Restricted stock During the three months ended September 30, 2021, the Company granted 2,500 restricted stock units under the 2021 Plan. These restricted stock units vest over a four-year period. A summary of the status and change in unvested restricted stock as of September 30, 2021 was as follows: Shares Weighted- Unvested as of December 31, 2020 537,633 $ 0.0028 Vested ( 403,225 ) $ 0.0028 Unvested as of September 30, 2021 134,408 $ 0.0028 At September 30, 2021 , there was less than $ 0.1 million of unrecognized stock-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted-average remaining vesting period of less than 1.0 year. 2021 Amended and Restated Employee Stock Purchase Plan In June 2021, the board of directors adopted, and the Company's stockholders approved, the 2021 Employee Stock Purchase Plan, or the ESPP, as amended and restated, which became effective on June 16, 2021. The Company has initially reserved 433,316 shares of common stock for sale under the ESPP. The aggregate number of shares reserved for sale under the ESPP will increase automatically on the first day of each fiscal year commencing on January 1, 2022 through January 1, 2031, by the number of shares equal to the least of (a) 1,083,290 shares, (b) 1 % of the total outstanding shares of common stock on such date, and (c) a number of shares as may be determined by the board of directors in any particular year. The first offering period under the ESPP commenced on June 16, 2021 and will end on December 13, 2021. As of September 30, 2021 , no shares have been purchased by employees under the ESPP.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	12. Net loss per share attributable to common stockholders The Company’s potential dilutive securities, which include convertible preferred stock, unvested restricted stock and common stock options, have been excluded from the computation of diluted net loss per share as the effects would be anti-dilutive. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at period end, from the computation of diluted net loss per share attributable to common stockholders for the period indicated because including them would have had an anti-dilutive effect: As of September 30, 2021 2020 Convertible preferred stock - 19,387,544 Unvested restricted stock 136,908 672,042 Outstanding options to purchase common stock 5,905,521 3,780,799 Total 6,042,429 23,840,385 As part of the license agreements with Harvard and Broad, the Company is required to make success payments. The Company may elect to make these payments by issuing shares of the Company's common stock. As of September 30, 2021, an aggregate of $ 6.3 million was payable relating to these success payments. The Company has elected to make these payments in cash, which will be paid out during the three months ending December 31, 2021.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	13. Income taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	14. Related party transactions For the three and nine months ended September 30, 2021 and 2020, the Company made payments of $ 0.1 million and $ 0.2 million, respectively, to four of the founder shareholders for scientific consulting and other expenses. All five founders also vested a total of 403,225 shares of restricted stock for the nine months ended September 30, 2021. An executive of Beam is a board member of the Company. In October 2020, the Company and Beam entered into a materials exchange agreement wherein the parties agreed that Beam would provide certain mRNA, gRNA, and protein to the Company and that the Company would provide certain gRNAs to Beam at an agreed upon price per each material provided. For the three and nine months ended September 30, 2021 , the Company recognized $ 0.0 million and $ 0.2 million, respectively, as a reduction to research and development expense related to reimbursements received for materials sold to Beam. See Note 8, L icense agreements. An officer of the Company is affiliated with Massachusetts General Hospital (“MGH”) as a physician. In February 2019 and November of 2019, the Company entered into an Option License Agreement and Patent License Agreement, respectively, with MGH. As part of the agreement, the Company recognizes an annual license fee of $ 0.1 million. An executive of Broad was a board member of the Company. The board member resigned, effective May 2021. In March 2019, the Company simultaneously entered into the Harvard/Broad License Agreement and Broad License Agreement for certain base editing technologies pursuant to which the Company received exclusive, worldwide, sublicensable, royalty-bearing licenses under specified patent rights to develop and commercialize licensed products and nonexclusive, worldwide, sublicensable, royalty-bearing licenses under certain patent rights to research and develop licensed products. Additional consideration under the license agreements include antidilution rights and success payments. See Note 8, License agreements.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	15. S ubsequent events The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. In October 2021, the Company entered into a sublease for 11,931 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease will commence in December 2021 and runs until December 2022 . The Company has the option to extend the sublease for one extension term of three months by written notice not less than six months prior to the expiration of the sublease term. The annual rent for the subleased premises will be approximately $ 1.4 million. The Company will also be required to pay its proportional share of operating expenses. In October 2021, the Company entered into a license agreement with a third party to obtain a non-exclusive license to certain technology the Company is using for research and development. As consideration for the license and rights granted under the agreement, the Company is required to make a one-time, non-refundable, upfront payment of $ 0.8 million. The license agreement provides for specified development milestones relating to the licensed technology and requires the Company to make payments upon achievement of each milestone up to an aggregate amount of $ 10.0 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Cash, Cash Equivalents and Restricted Cash	Cash, cash equivalents and restricted cash Restricted cash represents collateral provided for a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate facilities. A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: September 30, September 30, (in thousands) 2021 2020 Cash and cash equivalents $ 148,134 $ 11,859 Restricted cash 5,237 677 Total cash, cash equivalents and restricted cash $ 153,371 $ 12,536
Recently Adopted Accounting Pronouncements	Recently adopted accounting pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), as subsequently amended, which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors), and replaces the existing guidance in ASC 840. The FASB has issued several updates to the standard which: (i) clarify how to apply certain aspects of the new standard; (ii) provide an additional transition method for adoption of the new standard; (iii) provide a practical expedient for certain lessor accounting; and (iv) amend certain narrow aspects of the guidance. The new standard requires the identification and classification of arrangements that are or contain a lease and requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine the recognition pattern of lease expense over the term of the lease. In addition, a lessee is required to record (i) a right-of-use asset and a lease liability on its balance sheet for all leases with accounting lease terms of more than 12 months regardless of whether it is an operating or finance lease and (ii) lease expense in its consolidated statement of operations for operating leases and amortization and interest expense in its consolidated statement of operations for financing leases. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases under ASC 840. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), which added an optional transition method that allows companies to adopt the standard as of the beginning of the year of adoption as opposed to the earliest comparative period presented. This guidance is effective for the Company for annual periods beginning after December 15, 2021, including interim periods within that fiscal year. Early adoption is permitted. Prior to January 1, 2021, the Company accounted for leases pursuant to ASC 840, Leases. At lease inception, the Company determined if an arrangement was an operating or capital lease. For operating leases, the Company recognized rent expense, inclusive of rent escalations, holidays and lease incentives, on a straight-line basis over the lease term. The difference between rent expense recorded and the amount paid was recorded as deferred rent. The Company presented lease incentives as deferred rent and amortized the incentives as a reduction to rent expense on a straight-line basis over the lease term. The Company classified deferred rent as current and noncurrent liabilities based on the portion of the deferred rent that was scheduled to mature within the next twelve months. The Company adopted Topic 842 during the quarter ended September 30, 2021, with an effective adoption date of January 1, 2021 , using the modified retrospective transition approach which uses the effective date, or January 1, 2021, as the date of initial application. As a result, prior periods are presented in accordance with the previous guidance in ASC 840. At contract inception, the Company determines if an arrangement is or contains a lease. A lease conveys the right to control the use of an identified asset for a period of time in exchange for consideration. If an arrangement is determined to be or contain a lease, the lease is assessed for classification as either an operating or finance lease at the lease commencement date, defined as the date on which the leased asset is made available for use by the Company, based on the economic characteristics of the lease. The Company has elected to apply the package of practical expedients requiring no reassessment of whether any expired or existing contracts are or contain leases, the lease classification of any expired or existing leases, or the capitalization of initial direct costs for any existing leases. As a result of the adoption of ASC 842, the Company recorded (i) an operating lease liability of $ 2.6 million determined using an incremental borrowing rate as of the effective adoption date and (ii) an operating lease right-of-use asset of $ 2.4 million, net of the unamortized balance of prepaid/accrued rent as of the transition date. There was no impact to the Company’s results of operations and cash flows from operations. The Company has elected the short-term lease exemption, which allows the Company to not recognize lease liabilities and right-of-use assets arising from lease arrangements with lease terms of twelve months or less. For each lease with a term greater than twelve months, the Company records a right-of-use asset and lease liability. A right-of-use asset represents the economic benefit conveyed to the Company by the right to use the underlying asset over the lease term. A lease liability represents the Company’s obligation to make lease payments under the arrangement. The Company measures its lease liabilities at lease commencement as the present value of the future lease payments in the contract using the rate implicit in the contract, when available. As an implicit rate has not historically been readily determinable, the Company uses an incremental borrowing rate measured as the rate at which the Company could borrow, on a fully collateralized basis, a commensurate loan in the same currency over a period consistent with the lease term at the commencement date. The incremental borrowing rate is determined based on the Company's synthetic secured rating using the ordered logit model as of the measurement date. The Company measures its right-of-use assets as the lease liability plus initial direct costs and prepaid lease payments, less lease incentives granted by the lessor. Components of a lease are split into three categories: lease components, non-lease components, and non-components. The fixed and in-substance fixed contract consideration (including any consideration related to non-components) are allocated, based on the respective relative fair values, to the lease components and non-lease components. The Company has elected to account for lease and associated non-lease components together as a single lease component for all underlying assets and allocate all of the contract consideration to the lease component only. The Company remeasures right-of-use assets and lease liabilities when a lease is modified, and the modification is not accounted for as a separate contract. A modification is accounted for as a separate contract if the modification grants the Company an additional right of use not included in the original lease arrangement and the increase in lease payments is commensurate with the additional right of use. The Company assesses its right-of-use assets for impairment in a manner consistent with its assessment for long-lived assets held and used in operations. The Company’s operating leases are presented in the condensed consolidated balance sheet as operating lease right-of-use assets, classified as noncurrent assets, and operating lease liabilities, classified as current and noncurrent liabilities. Operating lease expense is recognized on a straight-line basis over the lease term. Variable costs associated with a lease, such as maintenance and utilities, are not included in the measurement of the lease liabilities and right-of-use assets but rather are expensed when the events determining the amount of variable consideration to be paid have occurred.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: September 30, September 30, (in thousands) 2021 2020 Cash and cash equivalents $ 148,134 $ 11,859 Restricted cash 5,237 677 Total cash, cash equivalents and restricted cash $ 153,371 $ 12,536

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Schedule of Marketable Securities by Security	Marketable securities by security type consisted of the following: September 30, 2021 (in thousands) Amortized Gross Gross Fair U.S. treasury bills and notes $ 223,434 $ 3 $ ( 5 ) $ 223,432 U.S. agency securities 17,644 1 ( 1 ) 17,644 Total $ 241,078 $ 4 $ ( 6 ) $ 241,076 December 31, 2020 (in thousands) Amortized Gross Gross Fair U.S. treasury bills and notes $ 32,221 $ 3 $ - $ 32,224 U.S. agency securities 30,890 5 - 30,895 Total $ 63,111 $ 8 $ - $ 63,119

Property and equipment, net (Ta

Property and equipment, net (Tables)	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, net	Property and equipment, net, consist of the following: (in thousands) September 30, 2021 December 31, Lab equipment $ 7,458 $ 3,937 Leasehold improvements 266 259 Furniture and fixtures 566 481 Computer equipment 145 105 Total property and equipment 8,435 4,782 Less accumulated depreciation ( 1,861 ) ( 798 ) Property and equipment, net $ 6,574 $ 3,984

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Financial Instruments, Assets and Liabilities	The following tables set forth the fair value of the Company’s financial instruments by level within the fair value hierarchy: As of September 30, 2021 (in thousands) Fair Level 1 Level 2 Level 3 Assets Money market funds $ 113,663 $ 113,663 $ - $ - Marketable securities: U.S. treasury bills and notes 223,432 - 223,432 - U.S. agency securities 17,644 - 17,644 - Total assets $ 354,739 $ 113,663 $ 241,076 $ - Liabilities Success payment liability* 11,760 - - 11,760 Total liabilities $ 11,760 $ - $ - $ 11,760 * During the three months ended September 30, 2021, multiple success payments were triggered and $ 6.3 million was due to Harvard and Broad, which was recorded as a current liability at September 30, 2021. As of December 31, 2020 (in thousands) Fair Level 1 Level 2 Level 3 Assets Money market funds $ 6,724 $ 6,724 $ - $ - Marketable securities: U.S. treasury bills and notes 32,224 - 32,224 - U.S. agency securities 30,895 - 30,895 - Total assets $ 69,843 $ 6,724 $ 63,119 $ — Liabilities Success payment liability $ 2,806 $ - $ - $ 2,806 Antidilution rights liability 6,916 - - 6,916 Total liabilities $ 9,722 $ - $ - $ 9,722
Schedule of Fair Value Liabilities Measured on Recurring Basis	The primary inputs used in valuing (i) the success payments liability and (ii) the antidilution rights liability associated with the Company’s realization of a certain valuation threshold through either a sale of the Company’s preferred stock, an initial public offering, or a company sale at September 30, 2021 (only applicable to success payments liability) and December 31, 2020, were as follows: At At Fair value of common stock (per share) $ 47.00 $ 8.24 Equity volatility 77 % 105 % Cumulative probability of triggering event n/a 70 % Expected term (in years) n/a 0.50
Schedule of Reconciliation of Changes in Fair Value of Financial Instruments	The reconciliation of changes in the fair value of financial instruments based on Level 3 inputs for the nine months ended September 30, 2021 is as follows: (in thousands) Antidilution Success Total Balance at December 31, 2020 $ 6,916 $ 2,806 $ 9,722 Issuance of common stock ( 32,490 ) - ( 32,490 ) Changes in fair value 25,574 8,954 34,528 Balance at September 30, 2021 $ - $ 11,760 $ 11,760 The reconciliation of changes in the fair value of financial instruments based on Level 3 inputs for the nine months ended September 30, 2020 is as follows: (in thousands) Preferred Antidilution Success Total Balance at December 31, 2019 $ 9,571 $ 2,044 $ 419 $ 12,034 Issuance of Series A Preferred Stock ( 7,064 ) - - ( 7,064 ) Issuance of common stock - ( 487 ) - ( 487 ) Changes in fair value ( 2,507 ) 1,804 13 ( 690 ) Balance at September 30, 2020 $ - $ 3,361 $ 432 $ 3,793

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following: (in thousands) September 30, December 31, Employee compensation and related benefits $ 3,151 $ 1,636 Accrued external research and development expenses 3,904 4,827 Professional fees 648 303 Other 424 423 Total $ 8,127 $ 7,189

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Components of Operating Lease Cost	The components of operating lease cost were as follows: (in thousands) Three Months Ended September 30, 2021 Nine Months Ended September 30, 2021 Operating lease costs $ 495 $ 1,418 Variable lease costs 187 530 Total $ 682 $ 1,948
Supplemental Cash Flow Information Related to Operating Leases	Supplemental cash flow information related to operating leases was as follows: (in thousands) Nine Months Ended September 30, 2021 Cash paid for amounts included in the measurements of lease liabilities: Operating cash flows related to operating leases $ 1,492
Schedule of Future Minimum Lease Commitments under Non-Cancellable Leases	Future minimum commitments under non-cancellable leases as of September 30, 2021 were as follows: Years ending December 31, Amount (in thousands) Remainder of 2021 $ 529 2022 2,764 2023 9,668 2024 9,958 2025 10,257 Thereafter 75,605 Total lease payments $ 108,781 Less: interest ( 21 ) Present value of operating lease liabilities $ 108,760

Stock-based compensation (Table

Stock-based compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Summary of Stock Based Compensation Expense	Stock-based compensation expense recorded in the consolidated statements of operations and comprehensive loss is as follows: Three months ended September 30, Nine months ended September 30, (in thousands) 2021 2020 2021 2020 Research and development $ 1,285 $ 130 $ 2,306 $ 250 General and administrative 1,008 83 2,008 158 Total stock-based compensation expense $ 2,293 $ 213 $ 4,314 $ 408
Schedule of Stock Option Activity	The following table provides a summary of stock option activity during the nine months ended September 30, 2021: Number of Weighted Weighted Aggregate (2) Outstanding at December 31, 2020 3,888,823 $ 2.13 Granted 2,408,546 13.75 Exercised ( 352,212 ) 1.49 Forfeited ( 39,636 ) 2.56 Outstanding at September 30, 2021 5,905,521 $ 6.92 8.8 $ 237,545 Exercisable at September 30, 2021 1,348,988 $ 1.99 8.1 $ 60,722 Expected to vest after September 30, 2021 (1) 4,556,533 $ 8.38 9.0 $ 176,823 (1) This represents the number of unvested options outstanding as of September 30, 2021 that are expected to vest in the future. (2) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money as of September 30, 2021 .
Schedule of Status and Change in Unvested Restricted Stock	A summary of the status and change in unvested restricted stock as of September 30, 2021 was as follows: Shares Weighted- Unvested as of December 31, 2020 537,633 $ 0.0028 Vested ( 403,225 ) $ 0.0028 Unvested as of September 30, 2021 134,408 $ 0.0028

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Potential Common Shares, Based on Amounts Outstanding at Period End were Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential common shares, presented based on amounts outstanding at period end, from the computation of diluted net loss per share attributable to common stockholders for the period indicated because including them would have had an anti-dilutive effect: As of September 30, 2021 2020 Convertible preferred stock - 19,387,544 Unvested restricted stock 136,908 672,042 Outstanding options to purchase common stock 5,905,521 3,780,799 Total 6,042,429 23,840,385

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Jun. 21, 2021	Sep. 30, 2021	Sep. 30, 2020
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Proceeds from initial public offering, net of underwriting discount		$ 285,214	$ 0
Cash, cash equivalents, and marketable securities		$ 389,200
IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Reverse stock split		one-for-9.2595
Shares issued and sold	16,141,157
Public offering price per share	$ 19
Proceeds from initial public offering, net of underwriting discount	$ 306,700
Net proceeds from sale of common stock	$ 281,600
Shares issued upon conversion of preferred stock	27,720,923	27,720,923
Underwriters Option
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued and sold	2,105,368

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Cash and cash equivalents	$ 148,134	$ 8,993	$ 11,859
Restricted cash	5,237		677
Total cash, cash equivalents and restricted cash	$ 153,371	$ 9,456	$ 12,536	$ 3,221

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Operating lease liability	$ 108,760
Operating lease right-of-use assets	$ 2,335	$ 0
ASC 842
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Operating lease liability	$ 2,600
Operating lease right-of-use assets	$ 2,400
Change in accounting principle, accounting standards update, immaterial effect	true

Marketable securities - Schedul

Marketable securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Debt Securities, Available-for-sale, Amortized Cost, Total	$ 241,078	$ 63,111
Gross unrealized gains	4	8
Gross unrealized losses	(6)	0
Fair value	241,076	63,119
U.S. Treasury Bills and Notes [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-sale, Amortized Cost, Total	223,434	32,221
Gross unrealized gains	3	3
Gross unrealized losses	(5)	0
Fair value	223,432	32,224
U.S. Agency Securities [Member]
Marketable Securities [Line Items]
Debt Securities, Available-for-sale, Amortized Cost, Total	17,644	30,890
Gross unrealized gains	1	5
Gross unrealized losses	(1)	0
Fair value	$ 17,644	$ 30,895

Marketable securities - Additio

Marketable securities - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Remaining contractual maturities of marketable securities term	1 year	1 year

Property and Equipment, net - S

Property and Equipment, net - Schedule of Property and Equipment, net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Total property and equipment	$ 8,435	$ 4,782
Less accumulated depreciation	(1,861)	(798)
Property and equipment, net	6,574	3,984
Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	7,458	3,937
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	266	259
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	566	481
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 145	$ 105

Property and Equipment, net - A

Property and Equipment, net - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Depreciation	$ 0.4	$ 0.6	$ 1.1	$ 1.1

Fair value of financial instr_3

Fair value of financial instruments - Summary of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021		Dec. 31, 2020
Assets
Total assets	$ 354,739		$ 69,843
Liabilities
Total liabilities	11,760		9,722
Success Payment Liability
Liabilities
Total liabilities	11,760	[1]	2,806
Antidilution Rights Liability
Liabilities
Total liabilities			6,916
U.S. Treasury Bills and Notes
Assets
Total assets	223,432		32,224
U.S. Agency Securities
Assets
Total assets	17,644		30,895
Level 1
Assets
Total assets	113,663		6,724
Liabilities
Total liabilities	0		0
Level 1 | Success Payment Liability
Liabilities
Total liabilities	0	[1]	0
Level 1 | Antidilution Rights Liability
Liabilities
Total liabilities			0
Level 1 | U.S. Treasury Bills and Notes
Assets
Total assets	0		0
Level 1 | U.S. Agency Securities
Assets
Total assets	0		0
Level 2
Assets
Total assets	241,076		63,119
Liabilities
Total liabilities	0		0
Level 2 | Success Payment Liability
Liabilities
Total liabilities	0	[1]	0
Level 2 | Antidilution Rights Liability
Liabilities
Total liabilities			0
Level 2 | U.S. Treasury Bills and Notes
Assets
Total assets	223,432		32,224
Level 2 | U.S. Agency Securities
Assets
Total assets	17,644		30,895
Level 3
Assets
Total assets	0		0
Liabilities
Total liabilities	11,760		9,722
Level 3 | Success Payment Liability
Liabilities
Total liabilities	11,760	[1]	2,806
Level 3 | Antidilution Rights Liability
Liabilities
Total liabilities			6,916
Level 3 | U.S. Treasury Bills and Notes
Assets
Total assets	0		0
Level 3 | U.S. Agency Securities
Assets
Total assets	0		0
Money Market Funds
Assets
Total assets	113,663		6,724
Money Market Funds | Level 1
Assets
Total assets	113,663		6,724
Money Market Funds | Level 2
Assets
Total assets	0		0
Money Market Funds | Level 3
Assets
Total assets	$ 0		$ 0
[1]	During the three months ended September 30, 2021, multiple success payments were triggered and $ 6.3 million was due to Harvard and Broad, which was recorded as a current liability at September 30, 2021.

Fair value of financial instr_4

Fair value of financial instruments - Summary of Assets and Liabilities Measured at Fair Value (Parenthetical) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Success payment liability, current portion	$ 6,250	$ 0
Harvard and Board
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Success payment liability, current portion	$ 6,300

Fair value of financial instr_5

Fair value of financial instruments - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 6,700		$ 113,700
Common stock shares issued	3,123,424		48,215,812
Issuance of common stock, value	$ 3		$ 48
Success payment liability, current portion	$ 0		6,250
Other Income
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, payable		$ 100	700	$ 100
Other Expense
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, payable		100		100
Success payment liability			9,000
Harvard and Board
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, payable			10,000,000
Success payment liability			6,300
Success payment liability, current portion			6,300
Harvard | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, payable			$ 31,300
Broad License Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common stock shares issued			276,075
Fair value inputs probability of termination	90.00%
Antidilution Rights Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common stock shares issued					878,098
Issuance of common stock, value					$ 32,500
Antidilution Rights Liability | Other Expense
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in liabilities		$ 100	$ 25,600	$ 1,800

Fair value of financial instr_6

Fair value of financial instruments - Schedule of Fair Value Liabilities Measured on Recurring Basis (Details) - $ / shares	Dec. 31, 2020	Sep. 30, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of common stock (per share)	$ 8.24	$ 47
Cumulative probability of triggering event	70.00%
Equity Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity volatility	105.00%	77.00%
Expected Term
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Expected term (in years)	6 months

Fair value of financial instr_7

Fair value of financial instruments - Schedule of Reconciliation of Changes in Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 9,722	$ 12,034
Issuance of Series A Preferred Stock			(7,064)
Issuance of common stock	$ 281,584	(32,490)	(487)
Changes in fair value		34,528	(690)
Ending balance		11,760	3,793
Antidilution Rights Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance		6,916	2,044
Issuance of Series A Preferred Stock			0
Issuance of common stock		(32,490)	(487)
Changes in fair value		25,574	1,804
Ending balance		0	3,361
Success Payment Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance		2,806	419
Issuance of Series A Preferred Stock			0
Issuance of common stock		0	0
Changes in fair value		8,954	13
Ending balance		$ 11,760	432
Preferred Stock Tranche Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance			9,571
Issuance of Series A Preferred Stock			(7,064)
Issuance of common stock			0
Changes in fair value			(2,507)
Ending balance			$ 0

Accrued expenses - Schedule of

Accrued expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Employee compensation and related benefits	$ 3,151	$ 1,636
Accrued external research and development expenses	3,904	4,827
Professional fees	648	303
Other	424	423
Total	$ 8,127	$ 7,189

Leases - Additional Information

Leases - Additional Information (Details)	Aug. 19, 2021USD ($)ft²	Jul. 31, 2021USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Jan. 31, 2021ft²	Jun. 30, 2020ft²	Mar. 31, 2020ft²
Lessee, Lease, Description [Line Items]
Increase in operating lease liability			$ (1,419,000)	$ 0
Operating lease incremental borrowing rate			1.57%
Weighted average remaining lease term			1 year 2 months 12 days
Cambridge Lease
Lessee, Lease, Description [Line Items]
Lease area | ft²							16,843
Lease term							2 years
Lease expiration date			Nov. 30, 2021
Increase in Right-Of-Use assets		$ 800,000
Increase in operating lease liability		$ 800,000
Cambridge Lease | First Amendment
Lessee, Lease, Description [Line Items]
Lease area | ft²						2,980
Cambridge Lease | Second Amendment
Lessee, Lease, Description [Line Items]
Lease area | ft²					2,980
Premises
Lessee, Lease, Description [Line Items]
Lease area | ft²	104,933
Lease not yet commenced , lease term	10 years
Option to extend, lease not yet commenced	five years
Option to extend true/false, lease not yet commenced	true
Operating lease payment term, lease not yet commenced, terms and conditions			The Company’s obligation for the payment of base rent for the Premises begins 90 days after the Commencement Date (the “Rent Commencement Date”) and will initially be $0.8 million per month, which will increase by approximately 3% per annum.
Option to terminate, lease not yet commenced	true
Tenant improvements allowance	$ 21,000,000
Additional tenant improvements allowance	$ 5,200,000		$ 11,000
Improvement allowance interest rate	8.00%
Prepaid rent			800,000
Lease space build out cost prior to commencement			$ 1,400,000
Premises | Letter of Credit
Lessee, Lease, Description [Line Items]
Letter of credit issued	$ 4,800,000
Letter of credit reduced value	$ 3,500,000

Leases - Summary of Components

Leases - Summary of Components of Operating Lease Cost (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Lease, Cost [Abstract]
Operating lease costs	$ 495	$ 1,418
Variable lease costs	187	530
Total	$ 682	$ 1,948

Leases - Summary of Supplementa

Leases - Summary of Supplemental Cash Flow Information Related to Operating Leases (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Cash paid for amounts included in the measurements of lease liabilities [Abstract]
Operating cash flows related to operating leases	$ 1,492

Leases - Summary of Future Mini

Leases - Summary of Future Minimum Commitments under Non-Cancellable Operating Leases (Details) $ in Thousands	Sep. 30, 2021USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
Remainder of 2021	$ 529
2022	2,764
2023	9,668
2024	9,958
2025	10,257
Thereafter	75,605
Total lease payments	108,781
Less: interest	(21)
Present value of operating lease liabilities	$ 108,760

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended			9 Months Ended		10 Months Ended	12 Months Ended
	Apr. 30, 2019	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Oct. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expenses		$ 17,495,000		$ 7,618,000	$ 42,263,000	$ 19,795,000
Common stock shares issued		48,215,812			48,215,812			3,123,424
Issuance of common stock			$ 281,584,000		$ (32,490,000)	(487,000)
Issuance of common stock, value		$ 48,000			$ 48,000			$ 3,000
Broad License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock shares issued		276,075			276,075
Issuance of common stock					$ 300,000
Preferred stock financing partially satisfied									$ 100,000
Preferred stock financing were fully satisfied								$ 500,000
Common stock issued for partly satisfying preferred stock financing									$ 121,411,000
Common stock issued for fully satisfying preferred stock financing								187,867
Payment of milestone development					46,200,000
Payment of sales based milestone					108,000,000
Harvard and Board
Research And Development Arrangement Contract To Perform For Others [Line Items]
Success payment liability		$ 6,300,000			6,300,000
Verily License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expenses		0		0	0	1,500,000
Amount due as a result of termination		0			0
Beam License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expenses		0		0	200,000	200,000
Common stock shares issued	276,075
Issuance of common stock	$ 300,000
Payment of milestone development	11,300,000
Payment of sales based milestone	15,000,000
Reimbursement received related to reduction to research and development expense		0		200,000	200,000	300,000
Milestone related expenses					0
Beam License Agreement | Non-US
Research And Development Arrangement Contract To Perform For Others [Line Items]
Payment of milestone development	5,600,000
Payment of sales based milestone	$ 7,500,000
Acuitas Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expenses		$ 100,000		$ 600,000	700,000	$ 1,300,000
License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Non Refundable Upfront License Fee							$ 2,000,000
Clinical And Regulatory Milestone					9,800,000
Sales Based Milestone					9,500,000
Milestone related expenses					0
License Agreement | Broad License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Non-Refundable Upfront License Fees Paid					$ 300,000
Antidilution Rights | Broad License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock shares issued		878,098			878,098
Issuance of common stock, value		$ 32,500,000			$ 32,500,000

Preferred Stock Tranche Liabi_2

Preferred Stock Tranche Liability - Additional Information (Details) - Seires A Preferred Stock [Member] - Third tranche $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares
Class Of Stock [Line Items]
Preferred stock, shares authorized	49,749,167
Preferred stock, value per share | $ / shares	$ 0.598
Preferred stock, shares issued	49,749,167
Proceeds from issuance or sale of preferred stock | $	$ 29.8

Preferred and Common Stock - Ad

Preferred and Common Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended	9 Months Ended
	Jan. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Jun. 21, 2021	Dec. 31, 2020
Subsidiary Sale Of Stock [Line Items]
Proceeds from issuance of Preferred Stock, net			$ 93,759	$ 92,616
Common stock, shares authorized			200,000,000			255,000,000
Common stock shares issued			48,215,812			3,123,424
Common Stock, Par Value			$ 0.001			$ 0.001
Harvard and Board [Member]
Subsidiary Sale Of Stock [Line Items]
Common stock shares issued		878,098
IPO
Subsidiary Sale Of Stock [Line Items]
Proceeds from issuance of Preferred Stock, net			$ 281,600
Common stock shares issued		16,141,157
Additional shares of common stock		2,105,368
Public price		$ 19
Proceeds from common stock, gross		$ 306,700
Shares issued upon conversion of preferred stock			27,720,923		27,720,923
Preferred Stock [Member]
Subsidiary Sale Of Stock [Line Items]
Preferred stock, shares authorized		5,000,000
Preferred Stock [Member] | Series B Preferred Stock
Subsidiary Sale Of Stock [Line Items]
Preferred stock, shares issued	77,163,022
Preferred stock, value per share	$ 1.2182
Proceeds from issuance of Preferred Stock, net	$ 94,000
Transaction costs on issuance	$ 200
Common Stock
Subsidiary Sale Of Stock [Line Items]
Common stock, shares authorized		200,000,000
Common Stock, Par Value		$ 0.001

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Details) - USD ($)	Jan. 01, 2022	Sep. 30, 2021	Jun. 16, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock reserved for future issuance		4,632,042
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost		$ 30,600
Weighted average period over which unrecognized compensation is expected to be recognized		3 years
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost		$ 100,000
Weighted average period over which unrecognized compensation is expected to be recognized		1 year
Twent Eighteen | Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock reserved for future issuance		6,885,653
2021 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock reserved for future issuance			3,466,530
Share-based Compensation Arrangement by Share-based Payment Award, Description		annual increase, to be added on the first day of each fiscal year, commencing on January 1, 2022 and continuing until, and including, January 1, 2031, equal to the lesser of (i) 5% of the number of shares of common stock outstanding on such date
Shares available for future grant		3,980,489
2021 Stock Incentive Plan | Common Stock | Forecast
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of outstanding stock, reserved for issuance	5.00%
2021 Stock Incentive Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future grant		2,500
Vesting period		4 years
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares of common stock reserved for future issuance		433,316
ESPP | Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of outstanding stock, reserved for issuance		1.00%
Increase in common stock capital shares, reserved for future issuance		1,083,290

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Research and development	$ 2,293	$ 213	$ 4,314	$ 408
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Research and development	1,285	130	2,306	250
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Research and development	$ 1,008	$ 83	$ 2,008	$ 158

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares
Share-based Payment Arrangement [Abstract]
Number of options, Outstanding, Beginning balance | shares	3,888,823
Number of options, Granted | shares	2,408,546
Number of options, Exercised | shares	(352,212)
Number of options, Forfeited | shares	(39,636)
Number of options, Outstanding, Ending balance | shares	5,905,521
Number of options, Exercisable | shares	1,348,988
Number of options, Expected to vest | shares	4,556,533	[1]
Weighted average exercise price, Beginning balance | $ / shares	$ 2.13
Weighted average exercise price, Granted | $ / shares	13.75
Weighted average exercise price, Exercised | $ / shares	1.49
Weighted average exercise price, Forfeited | $ / shares	2.56
Weighted average exercise price, Ending balance | $ / shares	6.92
Weighted average exercise price, Exercisable | $ / shares	1.99
Weighted average exercise price, Expected to vest | $ / shares	$ 8.38	[1]
Weighted average remaining contractual life (in years), Outstanding	8 years 9 months 18 days
Weighted average remaining contractual life (in years), Exercisable	8 years 1 month 6 days
Weighted average remaining contractual life (in years), Expected to vest	9 years	[1]
Aggregate intrinsic value, Outstanding | $	$ 237,545	[2]
Aggregate intrinsic value, Exercisable | $	60,722	[2]
Aggregate intrinsic value, Expected to vest | $	$ 176,823	[1],[2]
[1]	This represents the number of unvested options outstanding as of September 30, 2021 that are expected to vest in the future.
[2]	The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money as of September 30, 2021 .

Stock-based compensation - Su_2

Stock-based compensation - Summary of the Restricted Stock Units (Details) - RSU	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested RSUs, beginning balance, Unvested | shares	537,633
Unvested RSUs, Vested | shares	(403,225)
Unvested RSUs, ending balance, unvested | shares	134,408
Weighted-Average Grant Date Fair Value Per Share, beginning balance, Unvested | $ / shares	$ 0.0028
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares	0.0028
Weighted-Average Grant Date Fair Value Per Share, ending balance, Unvested | $ / shares	$ 0.0028

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Potential Common Shares, Based on Amounts Outstanding at Period End were Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	6,042,429	23,840,385
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	0	19,387,544
Unvested Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	136,908	672,042
Outstanding Options To Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities	5,905,521	3,780,799

Net Loss Per Share Attributab_4

Net Loss Per Share Attributable to Common Stockholders - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Success payment liability	$ 5,510	$ 2,806
License Agreement [Member] | Harvard and Board [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Success payment liability	$ 6,300

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 100	$ 100	$ 200	$ 200
Vesting of restricted common stock (in shares)			403,225
Research and development expenses	17,495	7,618	$ 42,263	19,795
Beam License Agreement
Related Party Transaction [Line Items]
Research and development expenses	$ 0	$ 0	200	$ 200
Officer [Member]
Related Party Transaction [Line Items]
Revenue from related parties			$ 100

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) $ in Millions	1 Months Ended	9 Months Ended
	Oct. 31, 2021USD ($)ft²	Sep. 30, 2021USD ($)
License Agreement
Subsequent Event [Line Items]
Milestone related expenses		$ 0
Subsequent Event | Cambridge, Massachusetts
Subsequent Event [Line Items]
Lease area | ft²	11,931
Sublease expiration date	2022-12
Lease Commitment Amount	$ 1.4
Subsequent Event | Cambridge, Massachusetts | Minimum
Subsequent Event [Line Items]
Lease term remaining	3 months
Subsequent Event | Cambridge, Massachusetts | Maximum
Subsequent Event [Line Items]
Lease term remaining	6 months
Subsequent Event | License Agreement
Subsequent Event [Line Items]
Non-refundable upfront payment	$ 0.8
Subsequent Event | License Agreement | Maximum
Subsequent Event [Line Items]
Milestone related expenses	$ 10