Omega Therapeutics (OMGA) 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. 04, 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	Omega Therapeutics, Inc.
Entity Central Index Key	0001850838
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		47,774,939
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Title of 12(b) Security	Common stock, par value $0.001 per share
Trading Symbol	OMGA
Security Exchange Name	NASDAQ
Entity File Number	001-40657
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-3247585
Entity Address, Address Line One	20 Acorn Park Drive
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02140
City Area Code	617
Local Phone Number	949-4360
Document Quarterly Report	true
Document Transition Report	false

Condensed Balance Sheets (Unaud

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 234,275	$ 22,951
Prepaid expenses and other current assets	4,484	1,052
Total current assets	238,759	24,003
Property and equipment, net	3,785	3,482
Restricted cash	341	341
Other assets	386	257
Total assets	243,271	28,083
Current liabilities:
Accounts payable	911	1,063
Accrued expenses	8,711	3,277
Other current liabilities	187	359
Long-term debt, current portion	4,500	3,000
Total current liabilities	14,309	7,699
Long-term debt, net	7,485	8,732
Other liabilities	1,138	1,055
Total liabilities	22,932	17,486
Commitments and contingencies (Note 8)
Redeemable convertible preferred stock:
Redeemable convertible preferred stock		75,225
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized as of September 30, 2021 and December 31, 2020, respectively; no shares issued and outstanding as of September 30, 2021 and December 31, 2020	0	0
Common stock, $0.001 par value; 200,000,000 and 137,700,000 shares authorized as of September 30, 2021 and December 31, 2020, respectively; 47,768,565 and 4,465,351 issued and outstanding as of September 30, 2021 and December 31, 2020, respectively	48	5
Additional paid-in capital	333,942	1,592
Accumulated deficit	(113,651)	(66,225)
Total stockholders' equity (deficit)	220,339	(64,628)
Total liabilities, redeemable convertible preferred stock, and stockholders' equity (deficit)	243,271	28,083
Series A Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	0	26,708
Series B Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	0	48,517
Series C Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	$ 0	$ 0

Condensed Balance Sheets (Una_2

Condensed Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Redeemable convertible preferred stock, shares authorized		107,125,232
Redeemable convertible preferred stock, shares issued		89,175,231
Redeemable convertible preferred stock, shares outstanding		89,175,231
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	137,700,000
Common stock, shares issued	47,768,565	4,465,351
Common stock, shares outstanding	47,768,565	4,465,351
Series A Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock, shares authorized	0	57,125,232
Redeemable convertible preferred stock, shares issued	0	56,775,232
Redeemable convertible preferred stock, shares outstanding	0	56,775,232
Series B Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock, shares authorized	0	50,000,000
Redeemable convertible preferred stock, shares issued	0	32,399,999
Redeemable convertible preferred stock, shares outstanding	0	32,399,999
Series C Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock, shares authorized	0	0
Redeemable convertible preferred stock, shares issued	0	0
Redeemable convertible preferred stock, shares outstanding	0	0

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - 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	$ 12,289	$ 5,505	$ 33,222	$ 13,921
General and administrative	4,459	2,047	10,911	4,422
Related party expense, net	473	493	1,235	1,060
Total operating expenses	17,221	8,045	45,368	19,403
Loss from operations	(17,221)	(8,045)	(45,368)	(19,403)
Other expense, net:
Interest expense, net	(339)	(196)	(741)	(584)
Change in fair value of warrant liability	(970)	1	(1,310)	5
Other income (expense), net	2	(1)	(7)
Total other expense, net	(1,307)	(196)	(2,058)	(579)
Net loss and comprehensive loss	$ (18,528)	$ (8,241)	$ (47,426)	$ (19,982)
Net loss per common stock attributable to common stockholders, basic and diluted	$ (0.57)	$ (1.97)	$ (3.41)	$ (4.98)
Weighted-average common stock used in net loss per share attributable to common stockholders, basic and diluted	32,303,540	4,180,696	13,898,089	4,014,800

Condensed Statements of Redeema

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Preferred Stock - Series A	Preferred Stock - Series B	Preferred Stock - Series C	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2019	$ (35,919)				$ 5	$ 854	$ (36,778)
Balance (in shares) at Dec. 31, 2019		56,775,232
Balance at Dec. 31, 2019		$ 26,708
Balance (in shares) at Dec. 31, 2019					3,775,292
Issuance of redeemable convertible preferred stock, shares			24,066,666
Issuance of redeemable convertible preferred stock			$ 36,038
Issuance of common stock for options exercised	7					7
Issuance of common stock for options exercised, shares					15,588
Vesting of restricted stock, shares					124,080
Stock-based compensation	145					145
Net loss	(5,418)						(5,418)
Balance at Mar. 31, 2020	(41,185)				$ 5	1,006	(42,196)
Balance (in shares) at Mar. 31, 2020		56,775,232	24,066,666
Balance at Mar. 31, 2020		$ 26,708	$ 36,038
Balance (in shares) at Mar. 31, 2020					3,914,960
Balance at Dec. 31, 2019	(35,919)				$ 5	854	(36,778)
Balance (in shares) at Dec. 31, 2019		56,775,232
Balance at Dec. 31, 2019		$ 26,708
Balance (in shares) at Dec. 31, 2019					3,775,292
Net loss	(19,982)
Balance at Sep. 30, 2020	(55,365)				$ 5	1,390	(56,760)
Balance (in shares) at Sep. 30, 2020		56,775,232	32,399,999
Balance at Sep. 30, 2020		$ 26,708	$ 48,517
Balance (in shares) at Sep. 30, 2020					4,253,837
Balance at Mar. 31, 2020	(41,185)				$ 5	1,006	(42,196)
Balance (in shares) at Mar. 31, 2020		56,775,232	24,066,666
Balance at Mar. 31, 2020		$ 26,708	$ 36,038
Balance (in shares) at Mar. 31, 2020					3,914,960
Issuance of redeemable convertible preferred stock, shares			3,333,333
Issuance of redeemable convertible preferred stock			$ 4,989
Issuance of common stock for options exercised	11					11
Issuance of common stock for options exercised, shares					25,933
Vesting of restricted stock, shares					124,080
Common stock repurchased	(1)					(1)
Common stock repurchased, shares					(2,564)
Stock-based compensation	107					107
Net loss	(6,323)						(6,323)
Balance at Jun. 30, 2020	(47,391)				$ 5	1,123	(48,519)
Balance (in shares) at Jun. 30, 2020		56,775,232	27,399,999
Balance at Jun. 30, 2020		$ 26,708	$ 41,027
Balance (in shares) at Jun. 30, 2020					4,062,409
Issuance of redeemable convertible preferred stock, shares			5,000,000
Issuance of redeemable convertible preferred stock			$ 7,490
Issuance of common stock for options exercised	38					38
Issuance of common stock for options exercised, shares					67,348
Vesting of restricted stock, shares					124,080
Stock-based compensation	229					229
Net loss	(8,241)						(8,241)
Balance at Sep. 30, 2020	(55,365)				$ 5	1,390	(56,760)
Balance (in shares) at Sep. 30, 2020		56,775,232	32,399,999
Balance at Sep. 30, 2020		$ 26,708	$ 48,517
Balance (in shares) at Sep. 30, 2020					4,253,837
Balance at Dec. 31, 2020	$ (64,628)				$ 5	1,592	(66,225)
Balance (in shares) at Dec. 31, 2020	89,175,231	56,775,232	32,399,999	0
Balance at Dec. 31, 2020	$ 75,225	$ 26,708	$ 48,517	$ 0
Balance (in shares) at Dec. 31, 2020	4,465,351
Issuance of redeemable convertible preferred stock, shares				41,833,328
Issuance of redeemable convertible preferred stock				$ 125,368
Issuance of common stock for options exercised	$ 45					45
Issuance of common stock for options exercised, shares					80,460
Stock-based compensation	194					194
Net loss	(13,488)						(13,488)
Balance at Mar. 31, 2021	(77,877)				$ 5	1,831	(79,713)
Balance (in shares) at Mar. 31, 2021		56,775,232	32,399,999	41,833,328
Balance at Mar. 31, 2021		$ 26,708	$ 48,517	$ 125,368
Balance (in shares) at Mar. 31, 2021					4,545,811
Balance at Dec. 31, 2020	$ (64,628)				$ 5	1,592	(66,225)
Balance (in shares) at Dec. 31, 2020	89,175,231	56,775,232	32,399,999	0
Balance at Dec. 31, 2020	$ 75,225	$ 26,708	$ 48,517	$ 0
Balance (in shares) at Dec. 31, 2020	4,465,351
Net loss	$ (47,426)
Balance at Sep. 30, 2021	220,339				$ 48	333,942	(113,651)
Balance (in shares) at Sep. 30, 2021		0	0	0
Balance at Sep. 30, 2021		$ 0	$ 0	$ 0
Balance (in shares) at Sep. 30, 2021					47,768,565
Balance at Mar. 31, 2021	(77,877)				$ 5	1,831	(79,713)
Balance (in shares) at Mar. 31, 2021		56,775,232	32,399,999	41,833,328
Balance at Mar. 31, 2021		$ 26,708	$ 48,517	$ 125,368
Balance (in shares) at Mar. 31, 2021					4,545,811
Issuance of common stock for options exercised	118					118
Issuance of common stock for options exercised, shares					101,224
Stock-based compensation	685					685
Net loss	(15,410)						(15,410)
Balance at Jun. 30, 2021	(92,484)				$ 5	2,634	(95,123)
Balance (in shares) at Jun. 30, 2021		56,775,232	32,399,999	41,833,328
Balance at Jun. 30, 2021		$ 26,708	$ 48,517	$ 125,368
Balance (in shares) at Jun. 30, 2021					4,647,035
Issuance of common stock from initial public offering, net of issuance costs, shares					8,300,976
Issuance of common stock from initial public offering, net of issuance costs	128,112				$ 8	128,104
Conversion of redeemable convertible preferred stock to common stock upon closing of initial public offering, shares		(56,775,232)	(32,399,999)	(41,833,328)
Conversion of redeemable convertible preferred stock to common stock upon closing of initial public offering		$ (26,708)	$ (48,517)	$ (125,368)
Conversion of redeemable convertible preferred stock to common stock upon closing of initial public offering, shares					34,678,733
Conversion of redeemable convertible preferred stock to common stock upon closing of initial public offering	200,594				$ 35	200,559
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	1,434					1,434
Exercise of warrants					82,193
Issuance of common stock for options exercised	88					88
Issuance of common stock for options exercised, shares					59,628
Stock-based compensation	1,123					1,123
Net loss	(18,528)						(18,528)
Balance at Sep. 30, 2021	$ 220,339				$ 48	$ 333,942	$ (113,651)
Balance (in shares) at Sep. 30, 2021		0	0	0
Balance at Sep. 30, 2021		$ 0	$ 0	$ 0
Balance (in shares) at Sep. 30, 2021					47,768,565

Condensed Statements of Redee_2

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Preferred Stock - Series B
Stock issuance costs			$ 10	$ 11	$ 62
Preferred Stock - Series C
Stock issuance costs		$ 132
Initial Public Offering
Stock issuance costs	$ 13,005

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating activities
Net loss			$ (47,426)	$ (19,982)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	$ 400	$ 300	1,003	715
Amortization of debt issuance costs and debt discount			89	36
Change in fair value of warrant liability	970	(1)	1,310	(5)
Change in fair value of success fee obligation			6
Non-cash interest expense			164
Stock-based compensation expense			2,002	481
Deferred rent			(107)	(82)
Loss on disposal of equipment			(3)	(1)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets			(3,429)	(261)
Other assets			(129)
Accounts payable			(151)	(126)
Accrued expenses and other current liabilities			4,681	1,392
Other liabilities			337
Net cash used in operating activities			(41,653)	(17,833)
Investing activities
Purchases of property and equipment			(1,267)	(1,285)
Net cash used in investing activities			(1,267)	(1,285)
Financing activities
Proceeds from issuance of redeemable convertible preferred stock			125,500	48,600
Payments for preferred stock issuance costs			(132)	(83)
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts			131,238
Payments for initial public offering costs			(2,413)
Payment of success fee			(200)
Proceeds from exercise of stock options			251	55
Net cash provided by financing activities			254,244	48,572
Net increase in cash, cash equivalents and restricted cash			211,324	29,454
Cash, cash equivalents and restricted cash—beginning of period			23,292	2,615
Cash, cash equivalents and restricted cash—end of period	234,616	32,069	234,616	32,069
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	234,275	31,728	234,275	31,728
Restricted cash	341	341	341	341
Cash, cash equivalents and restricted cash—end of period	$ 234,616	$ 32,069	234,616	32,069
Supplemental disclosures of cash flow information
Cash paid for interest			532	550
Supplemental disclosure of noncash investing and financing activities
Conversion of preferred stock to common stock			200,594
Reclassification of warrants to additional paid-in capital			1,434
Deferred offering costs in accounts payable and accrued expenses			713
Purchase of property and equipment included accounts payable and accrued expenses			$ 40	$ 491

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Nature Of Business And Basis Of Presentation [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Ba sis of Presentation Organization Omega Therapeutics, Inc. (the “Company” or “Omega”) is a development-stage biotechnology company pioneering a new systematic approach to use mRNA therapeutics as programmable epigenetic medicines by leveraging its OMEGA Epigenomic Programming platform (“OMEGA platform”). The OMEGA platform harnesses the power of epigenetics, the mechanism that controls gene expression and every aspect of an organism’s life from cell genesis, growth and differentiation to cell death. The OMEGA platform enables control of fundamental epigenetic processes to correct the root cause of disease by returning aberrant gene expression to a normal range without altering native nucleic acid sequences. The Company was incorporated in July 2016 (“inception”) as a Delaware corporation and its offices are in Cambridge, Massachusetts. Liquidity Since its inception, the Company has devoted substantially all of the resources to building its platform and advancing development of its portfolio of programs, establishing and protecting its intellectual property, conducting research and development activities, organizing and staffing the Company, business planning, raising capital and providing general and administrative support for these operations. 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. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. In August 2021, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold 8,300,976 shares of its common stock, including 900,976 shares pursuant to the partial exercise of the underwriters’ option to purchase additional shares, at a public offering price of $ 17.00 per share, for aggregate gross proceeds of $ 141.1 million. The Company received approximately $ 128.1 million in net proceeds after deducting underwriting discounts and commissions and other offering expenses payable by the Company. As presented in the unaudited condensed financial statements, the Company had cash and cash equivalents of $ 234.3 million at September 30, 2021. The Company expects that its cash and cash equivalents, including the net proceeds from its IPO, will enable it to fund its operating expenses and capital expenditure requirements for at least twelve months from the filing date of this Quarterly Report on Form 10-Q. However, additional funding will be necessary beyond this point to fund future preclinical and clinical activities and to develop new product candidates. The Company expects to finance its future cash needs through a combination of equity offerings, debt financings, marketing and distribution arrangements and other collaborations, strategic alliances and licensing arrangements, or other sources . COVID-19-Related Significant Risks and Uncertainties With the global COVID-19 pandemic continuing throughout 2021, the Company is following, and plans to continue to follow, recommendations from federal, state and local governments regarding workplace policies, practices and procedures. To provide a safe work environment for its employees, the Company has implemented various measures to promote social distancing, increase sanitization of its facilities and provide personal protective equipment for its employees. The Company expects to continue incurring additional costs to ensure it adheres to the guidelines instituted by the federal, state and local governments and to provide a safe working environment to its onsite employees. The extent to which the COVID-19 pandemic and variants of the virus impacts the Company’s business, its corporate development objectives, results of operations and financial condition, and the fair value of and market for its common stock, will depend on future developments that are highly uncertain and cannot be predicted with confidence at this time, such as the ultimate duration of the pandemic, travel restrictions, quarantines, social distancing and business closure requirements, and the effectiveness of actions taken globally to contain and treat the disease. Disruptions to the global economy, disruption of global healthcare systems, and other significant impacts of the COVID-19 pandemic could have a material adverse effect on the Company’s business, financial condition, results of operations and growth prospects. While the COVID-19 pandemic did not significantly impact the Company’s business or results of operations during the nine months ended September 30, 2021, the length and extent of the pandemic, its consequences, and containment efforts will determine the future impact on the Company’s operations and financial condition. Basis of Presentation The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). All amounts herein are expressed in U.S. dollars (“USD”) unless otherwise noted.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Unaudited Condensed Financial Information The accompanying unaudited condensed financial statements included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). The unaudited condensed financial statements have been prepared on the same basis as audited financial statements, except certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair representation of the results for the reported periods. These unaudited condensed financial statements should be read in conjunction with the audited financial statements and notes thereto contained in the Company’s final prospectus for its IPO dated August 2, 2021 and filed with the SEC pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Prospectus”). The results for the nine months ended September 30, 2021 are not necessarily indicative of results to be expected for the year ending December 31, 2021, any other interim periods, or any future year or period. Use of Estimates The preparation of the unaudited condensed financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these unaudited condensed financial statements include, but are not limited to, the selection of useful lives of property and equipment, the fair values of common stock, redeemable convertible preferred stock, and warrants prior to the IPO, the fair value of the success fee obligation, and stock-based compensation. Actual results could differ from these estimates. Changes in estimates are reflected in reported results in the period in which they become known. Summary of Significant Accounting Policies The Company’s significant accounting policies are described in Note 2, “Summary of significant accounting policies,” to the Company’s audited financial statements included in the Prospectus. Except as detailed below, there have been no material changes to the significant accounting policies during the nine months ended September 30, 2021. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ deficit as a reduction of proceeds generated as a result of the offering. Should a planned equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the unaudited condensed statement of operations. Upon closing of the IPO in August 2021, deferred offering costs were derecognized and recorded against the IPO proceeds as a reduction to additional paid-in capital. There were no deferred offering costs as of September 30, 2021 . Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight- line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. As the Company has elected to use the extended transition period for complying with new or revised accounting standards as available under the Jumpstart Our Business Startups Act (“JOBS Act”), the standard is effective for the Company beginning January 1, 2022, with early adoption permitted. The Company engaged an external third party to assist with the adoption of ASU 2016-02 and is currently assessing the impact it will have on its financial statements. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes (“ASU 2019-12”). ASU 2019-12 eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new guidance also simplifies aspects of the accounting for franchise taxes, enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. Adoption of the standard requires certain changes to be made prospectively and certain others to be made retrospectively. As the Company has elected to use the extended transition period for complying with new or revised accounting standards as available under the JOBS Act, the standard is effective for the Company beginning January 1, 2022, with early adoption permitted. The Company does not expect ASU 2019-12 to have a material effect on the Company’s financial statements.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2021
Prepaid Expense And Other Assets Current [Abstract]
Prepaid Expenses and Other Current Assets	3. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): September 30, December 31, 2021 2020 Prepaid software $ 127 $ 78 Prepaid research and development 1,355 653 Prepaid other 507 120 Prepaid insurance 2,128 8 Other receivables 367 193 Prepaid expenses and other current assets $ 4,484 $ 1,052

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net consists of the following (in thousands): September 30, December 31, 2021 2020 Leasehold improvements $ 1,378 $ 1,378 Lab equipment 4,532 3,444 Furniture and fixtures 1,047 985 Computer equipment 129 129 Construction in process 165 16 Total property and equipment 7,251 5,952 Less accumulated depreciation ( 3,466 ) ( 2,470 ) Property and equipment, net $ 3,785 $ 3,482 Depreciation expense for the three months ended September 30, 2021 and 2020 was $ 0.4 million and $ 0.3 million, respectively. Depreciation expense for the nine months ended September 30, 2021 and 2020 was $ 1.0 million and $ 0.7 million, respectively.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2021
Accrued Liabilities Current [Abstract]
Accrued Expenses	5. Accrued Expenses Accrued expenses consist of the following (in thousands): September 30, December 31, 2021 2020 Employee related expenses $ 1,771 $ 1,124 Research costs 5,082 1,724 Professional and consulting fees 1,403 278 Interest 58 44 Other 397 107 Total $ 8,711 $ 3,277

Term Loan

Term Loan	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Term Loan	6. Term Loan On March 9, 2018 (“Closing Date”), the Company entered into the loan and security agreement (“Loan Agreement”) with Pacific Western Bank (“PWB”), a California state-chartered bank. Under the Loan Agreement, the Company may borrow amounts not to exceed $ 8.0 million, consisted of Tranche I and Tranche II, with the Tranche I funded on the Closing Date and Tranche II funded no later than 18 months from the Closing Date . The Company borrowed $ 3.5 million under Tranche I on the Closing Date and $ 4.5 million under Tranche II in August 2018, and the full amount of $ 8.0 million was to be repaid beginning 18 months from the Closing Date in thirty equal installments, including interest at a floating annual rate equal to the greater of (i) 0.75 % above the prime rate then in effect and (ii) 5.00 %, due monthly starting the first month after the Closing Date. In conjunction with the Loan Agreement, under Tranche I, the Company issued a warrant to PWB to purchase 87,500 shares of Series A redeemable convertible preferred stock (“Series A Preferred Stock”) at the initial strike price of $ 0.5 0 per share. The warrant is exercisable for a 10-year period. Additionally, the warrant shall be exercisable for an additional number of shares of Series A Preferred Stock equal to the amount borrowed under Tranche II multiplied by 0.0125 . In no event shall the warrant be exercisable for more than 200,000 shares of Series A Preferred Stock. Upon closing of Tranche II, the warrant was not exercised for additional number of shares of Series A Preferred Stock. In lieu of exercising the warrant, PWB may, in whole or in part, convert the warrant into a number of shares of Series A Preferred Stock, determined by (a) dividing the aggregate fair market value of the Series A Preferred Stock shares minus the aggregate warrant price of such shares by (b) the fair market value of one share of Series A Preferred Stock. The fair market value of the Series A Preferred Stock shares shall be determined based upon either the publicly traded closing price on the date of the conversion or, if not publicly traded, a value deemed appropriate by the Company’s board of directors. Refer to Note 7, Fair value of financial instruments , for further discussion on the valuation methodology and inputs for the determination of the fair value of the warrants. On September 30, 2019, the Company entered into an amendment to the Loan Agreement (the “First Amendment”), in which PWB made an additional term loan pursuant to a new Tranche III to the Company in an aggregate principal amount of $ 12.0 million. The proceeds of the term loan pursuant to Tranche III were first applied to the repayment in full of all outstanding principal and accrued interest on the outstanding term loan of $ 8.0 million borrowed pursuant to Tranche I and Tranche II. The remaining cash proceeds of $ 4.0 million was used for general working capital and for capital expenditures purposes. The maturity date of the additional term loan was initially March 9, 2022 , and it would have been repaid beginning on January 9, 2020 in twenty-seven equal installments. However, the closing of the Company’s Series B redeemable convertible preferred stock (“Series B Preferred Stock”) financing in January 2020 satisfied the cash proceeds milestone noted in the First Amendment, in which the maturity date of the amended term loan was extended to June 9, 2023 , and the term loan was to be repaid beginning in January 2021 in thirty equal installments, including interest at a floating annual rate equal to the greater of (i) 0.75 % above the prime rate then in effect and (ii) 6.00 %, due monthly starting the first month after September 30, 2019. The Company incurred $ 15 thousand of debt issuance costs, which was recorded as a direct reduction against the additional term loan and amortized over the life of the associated term loan as a component of interest expense using the effective interest method. In conjunction with the First Amendment, the Company also issued a warrant to purchase 350,000 shares of Series A Preferred Stock, which effectively restated and replaced the original warrant agreement. The strike price of the amended warrant is $ 0.50 per share, and the term remains unchanged, expiring in March 2028 . Refer to Note 7, Fair value of financial instruments , for further discussion on the valuation methodology and inputs for the determination of the fair value of the warrants. As the warrants issued were freestanding financial instruments that were exercisable for contingently redeemable shares, they were initially recorded at fair value on the date of issuance as a liability, with a corresponding discount recorded against the face value of the term loan. The discount was accreted against the face value of the term loan over its remaining term as additional interest expense. The change in estimated fair value of the warrants during the period was remeasured at each reporting date and recognized as a component of other income (expense), net in the statements of operations and comprehensive loss. Upon the closing of the IPO, the Company’s outstanding warrants to purchase Series A Preferred Stock automatically became warrants to purchase an aggregate of 92,647 shares of common stock. As a result, the fair value of the warrants was reclassified to additional paid-in capital and, as of September 30, 2021 , the Company no longer had a warrant liability to remeasure. Additionally, the remaining unamortized debt discount of $ 0.1 million related to the warrants was written off during the three months ended September 30, 2021 . In August 2021, the holders of such warrants completed a cashless exercise of the warrants, resulting in the Company’s issuance of 82,193 shares of its common stock, whereby 10,454 shares of common stock were withheld by the Company to pay for the exercise price of the warrants. On January 22, 2020, the Loan Agreement was further amended (the “Second Amendment”) to extend the principal repayment start date, from January 9, 2020 as noted in the First Amendment to February 9, 2020. The Loan Agreement was further amended on December 30, 2020 (the “Third Amendment”) to extend the principal repayment date. The maturity date of the term loan was extended to June 30, 2023 , and it was to be repaid beginning on June 30, 2021 in twenty-four equal installments, including interest at a floating annual rate equal to the greater of (i) 0.75 % above the prime rate then in effect and (ii) 6.00 %, due monthly starting the first month after December 30, 2020. As a result of the closing of Series C redeemable convertible preferred stock (“Series C Preferred Stock”) in March 2021, as further discussed in Note 10, Redeemable convertible preferred stock , the Company satisfied the cash proceeds milestone as defined in the Third Amendment, in which the Company received gross cash proceeds of more than $ 50.0 million from the issuance of new preferred stock prior to June 30, 2021. Accordingly, the principal repayment date of the term loan was further extended to December 31, 2021 and the maturity date was extended to December 31, 2023 . There are no other changes to the terms as a result of the achievement of the cash proceeds milestone. In connection with the Third Amendment, the Company incurred $ 15 thousand of debt issuance costs, which have been recorded as a direct reduction against the term loan and amortized over the life of the associated term loan as a component of interest expense using the effective interest method. Borrowings under the Loan agreement, as amended, are collateralized by substantially all of the Company’s personal property, other than its intellectual property. There are no financial covenants associated with the Loan Agreement, as amended; however, the Company is subject to certain affirmative and negative covenants to which the Company will remain subject until maturity. Additionally, the Company was required to pay a success fee of $ 0.2 million upon the occurrence of a specified liquidity event, including an IPO, as described in the Loan Agreement. The Company determined that this obligation represented a freestanding financial instrument. Accordingly, the success fee obligation was classified as a liability on the Company’s balance sheet as of December 31, 2020 and initially recorded at fair value, with changes in fair value for each reporting period recognized in other income (expense), net in the statements of operations and comprehensive loss. The fair value of such obligation was remeasured at the end of each reporting period until the liability was settled, for which it was settled and paid in August 2021 upon the completion of the IPO. As of September 30, 2021 and December 31, 2020 , the long-term debt, current portion was $ 4.5 million and $ 3.0 million, and the long-term debt was $ 7.5 million and $ 9.0 million, respectively. The Company’s outstanding term loan balance was comprised of the following (in thousands): September 30, December 31, 2021 2020 Principal $ 12,000 $ 12,000 Unamortized debt discount ( 15 ) ( 268 ) Net carrying amount $ 11,985 $ 11,732 The Company determined that the expected life of the debt was equal to the term on the term loan. The effective interest rate on the liability component ranged from 5.53 % to 7.51 % for the period from the date of issuance through September 30, 2021 . The following table sets forth total interest expense recognized related to the term loan (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Contractual interest expense $ 184 $ 184 $ 546 $ 548 Amortization of debt issuance costs and debt discount 179 12 253 36 Total interest expense $ 363 $ 196 $ 799 $ 584 At September 30, 2021 and December 31, 2020 , accrued interest on the term loan was $ 58 thousand and $ 44 thousand, respectively. The Company is required to repay the following principal amounts in connection with its term loan (in thousands): 2021 (remaining 3 months) $ — 2022 6,000 2023 6,000 $ 12,000

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	7. Fair Value of Financial Instruments As of September 30, 2021 , the fair value of the Company’s warrant liability and success fee obligation was zero as the warrants became warrants to purchase common stock and the success fee was paid in August 2021. As of December 31, 2020, the fair value of the Company’s financial instruments is summarized in the table below (in thousands): December 31, 2020 Level 1 Level 2 Level 3 Total Financial Liabilities Warrant liability $ — $ — $ 124 $ 124 Success fee obligation — — 194 194 Total $ — $ — $ 318 $ 318 The Company’s warrant liability and success fee obligation contained unobservable inputs that reflected the Company’s own assumptions in which there was little, if any, market activity at the measurement date. Accordingly, the Company’s warrant liability and success fee obligation were measured at fair value on a recurring basis using unobservable inputs at each reporting period, through the settlement date, and were classified as Level 3 inputs. As of December 31, 2020 , the warrant liability was recorded as other liabilities on the balance sheet as it was deemed more probable than not by the Company to be settled in longer than one year. The fair value of the warrants was estimated using the Black-Scholes option-pricing model. The expected term represented the remaining contractual term of the warrants. The risk-free interest rate was determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the remaining contractual term of the warrants. The expected dividend was zero as the Company had not paid any dividends on its common stock. The Company was a private company and lacked company-specific historical and implied volatility information of its stock. Therefore, it estimated its expected stock volatility based on the historical volatility of publicly traded peer companies for a term equal to the remaining contractual term of the warrants. The assumptions used in the Black-Scholes option-pricing model for the warrants were as follows: August 3, December 31, 2021 2020 Expected volatility 77.03 - 77.62 % 73.09 - 77.79 % Risk-free interest rate 0.95 - 1.40 % 0.58 - 0.65 % Expected dividend yield 0.00 % 0.00 % Expected term (in years) 6.6 - 6.9 7.2 - 7.9 The fair value of the warrants was remeasured at each reporting period, with changes in fair value recognized in the statements of operations and comprehensive loss, until the warrants were settled. The change in the fair value of the warrant liability during the three and nine months ended September 30, 2021 was $ 1.0 million and $ 1.3 million, respectively; and there was an immaterial change in the fair value of the warrant liability for both of the three and nine months ended September 30, 2020. The fair value of the success fee obligation was determined using the probability-weighted expected return method. The key estimates and assumptions impacting the fair value included the probability of achieving a specified liquidity event, the expected timing of achieving a liquidity event and discount rate. The fair value of the success fee obligation was remeasured at each reporting period, with changes in fair value recognized in the statement of operations and comprehensive loss, until such liability was settled. As a result of the completion of the IPO in August 2021, the Company remeasured the fair value of the success fee obligation to the full payment amount. The following table provides a roll-forward of the fair values of the Company’s warrant liability and the success fee obligation for which fair value is determined by Level 3 inputs (in thousands): Warrant Success Fair value at January 1, 2021 $ 124 $ 194 Change in fair value 1,310 6 Settlement of success fee obligation — ( 200 ) Reclassification to additional paid-in-capital in connection with IPO ( 1,434 ) — Fair value at September 30, 2021 $ — $ — Warrant Success Fair value at January 1, 2020 $ 127 $ — Change in fair value ( 5 ) — Fair value at September 30, 2020 $ 122 $ —

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Operating Leases In 2017, the Company entered a noncancelable operating lease agreement to lease its office space at 325 Vassar Street, Cambridge, Massachusetts, which will expire in September 2024 . The Company is required to pay property taxes, insurance, and normal maintenance costs. The operating lease contains predetermined fixed escalations of minimum rentals during the lease term. During 2018, the Company received $ 1.1 million of landlord-funded leasehold improvements related to the leased office space. The landlord-funded leasehold improvements were recorded as property and equipment, net and deferred rent in the balance sheets and are being amortized as a reduction to rent expense over the life of the lease. In 2019 and 2020, the Company entered into sublease agreements with two related parties to sublease this office and laboratory space. Refer to Note 15, Related party transactions , for further details. On July 13, 2020, the Company entered into a Shared Space Arrangement (“the Arrangement”) with Senda Biosciences, Inc. (“Senda”, also formerly known as Kintai Therapeutics, Inc.) to share one-third of Senda’s 69,867 square feet of leased space at 20 Acorn Park Drive, Cambridge, Massachusetts. Senda is a related party as it is an affiliate of Flagship Pioneering (“Flagship”). The Arrangement commenced on August 1, 2020 and continues through July 31, 2022 with two options to extend the term of the Arrangement for a period of 24 months each. The operating lease contains predetermined fixed escalations of minimum rentals during the lease term, and the Company is required to pay property taxes, insurance, and normal maintenance costs. During the three and nine months ended September 30, 2021, the Company incurred $ 0.6 million and $ 1.9 million for rental expenses. During the three and nine months ended September 30, 2020, the Company incurred $ 0.4 million for rental expenses. There was no outstanding balance due to Senda as of September 30, 2021 and December 31, 2020. The Company recognizes the rental expense on a straight-line basis over the life of the respective lease from the date the Company takes possession of the office and records the difference between amounts charged to operations and amounts paid as deferred rent. Rent expense under both lease agreements for the three months ended September 30, 2021 and 2020 was $ 0.8 million and $ 0.7 million, respectively; rent expense under both lease agreements for the nine months ended September 30, 2021 and 2020 was $ 2.5 million and $ 1.4 million, respectively. As of September 30, 2021, the future minimum lease payments for the Company’s facility operating leases were as follows (in thousands): 2021 (remaining 3 months) $ 839 2022 2,618 2023 1,563 2024 1,205 Total minimum lease payments $ 6,225

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2021
License Agreements [Abstract]
License Agreements	9. License Agreements Flagship Pioneering Innovations V, Inc. In March 2019, the Company entered into an exclusive license agreement with Flagship Pioneering Innovations V, Inc., an affiliate of Flagship, under which the Company was granted an exclusive, worldwide, royalty-bearing, sublicensable, transferable license under specified patent rights to develop, manufacture and commercialize licensed products (the “Flagship License”). Under the terms of the Flagship License, the Company is obligated to pay low single digit percentage royalties on net sales of licensed products by the Company. Royalties shall be paid by the Company on a country-by-country basis until expiration or abandonment of the last valid patent claim covering such licensed product in such country. The Company is also obligated to reimburse Flagship for patent prosecution costs. The royalty payment is contingent upon sales of licensed products under the Flagship License. As such, when such expense is considered probable and estimable at the commencement of sales, the Company will account for the royalty expense as cost of sales for the amount it is obligated. Whitehead Institute for Biomedical Research In May 2019, the Company entered into an exclusive license agreement with the Whitehead Institute for Biomedical Research (“WIBR”), an affiliate of one of the Company’s board members, under which the Company was granted an exclusive, worldwide, royalty-bearing, sublicensable license under specified patent rights to research, make, have made, use, sell, offer to sell, lease and import products and to perform and have performed licensed processes (the “WIBR Exclusive License”). Under the terms of the WIBR Exclusive License, the Company paid a nonrefundable upfront fee of less than $ 0.1 million upon the commencement of the exclusive license agreement. The Company is obligated to pay WIBR annual license maintenance fees of less than $ 0.1 million and low single digit percentage royalties on net sales of licensed products by the Company and its affiliates and sublicensees. Additionally, the Company is required to make milestone payments of up to $ 1.7 million in the aggregate for each of the first three licensed products (excluding backup products) upon the achievement of specified clinical, regulatory, and sublicensing milestones. In addition, the Company is required to pay to WIBR a percentage of the non-royalty payments that it receives from sublicensees of the WIBR Exclusive License. This percentage ranges from zero to low double-digits and will be based upon the stage of development of the licensed product at the time such sublicense is executed. In May 2019, the Company also entered into a co-exclusive license agreement with WIBR under which the Company was granted a co-exclusive, worldwide, royalty-bearing, sublicensable license under specified patent rights to research, make, have made, use, sell, offer to sell, lease and import products and to perform and have performed licensed processes (the “WIBR Co-Exclusive License”). Under the terms of the WIBR Co-Exclusive License, the Company paid a nonrefundable upfront fee of less than $ 0.1 million upon the commencement of the co-exclusive license agreement. The Company is obligated to pay WIBR annual license maintenance fees of less than $ 0.1 million and sub single digit percentage royalties on net sales of licensed products by the Company and its affiliates and sublicensees as well as low single digit percentage royalties on licensed service income received by the Company and its affiliates. Additionally, the Company is required to make milestone payments of up to $ 1.9 million in the aggregate for each of the first three licensed products (excluding backup products) upon the achievement of specified clinical and regulatory milestones. In addition, the Company is required to pay to WIBR annual fees of less than $ 0.1 million for each sublicense agreement. For both of the three-month periods ended September 30, 2021 and 2020 , the Company recognized expenses of less than $ 0.1 million for the reimbursable patent costs related to the licenses under the agreements. During the three months ended September 30, 2021 , upon the achievement of a milestone event in accordance with the exclusive license agreement, the Company made a milestone payment in the amount of less than $ 0.1 million. For both of the nine-month periods ended September 30, 2021 and 2020 , the Company recognized expenses for the license maintenances fees and the reimbursable patent costs in the amount of $ 0.1 million. There was no outstanding payment due to WIBR as of September 30, 2021 and December 31, 2020. The annual maintenance fees will be recorded as an expense on an annual basis based on the stated amount for the applicable year. Upon determination that a milestone payment is probable to occur, the amount due will be recorded as research and development. As mentioned above, a milestone event was achieved during the three months ended September 30, 2021, and the related milestone payment was recorded as research and development expense during the period. During the three and nine months ended September 30, 2020 , the triggering of such milestone payments was not considered probable; as such, no expense has been recorded for these milestones during these periods. Lastly, the royalty payments and the sublicense non-royalty payments are contingent upon sales of licensed products or execution of a sublicence agreement under the WIBR Exclusive and Co-Exclusive Licenses. As such, when such expenses are considered probable and estimable at the commencement of sales or execution of a sublicense agreement, the Company will accrue royalty expense and sublicence non-royalty payments, as applicable, for the amount the Company is obligated. Acuitas Therapeutics, Inc. In October 2020, the Company entered into a development and option agreement (the “Development and Option Agreement”) with Acuitas Therapeutics, Inc. (“Acuitas”). Under the terms of the Development and Option Agreement, the parties agreed to jointly develop certain products combining the Company’s gene modulating therapeutics with Acuitas’s lipid nanoparticles. Additionally, in accordance with the Development and Option Agreement, the Company has options to obtain non-exclusive, worldwide, sublicensable licenses under Acuitas’s patents and know-how related to lipid nanoparticle technology (“Acuitas LNP Technology”) with respect to two specified targets (e.g., OEC constructs) (“Reserved Targets”) to develop and commercialize one or more therapeutic products relating to such targets. For each option and Reserved Target, the Company is obligated to pay an annual technology access fee and target reservation and maintenance fees collectively in the low-mid six figures until such Reserved Target is removed from the Reserved Target list or until the Company exercises an option with respect to such Reserved Target. In the event that the Company exercises the options, the Company will pay $ 1.5 million for the first non-exclusive license and $ 1.75 million for the second non-exclusive license. Under the terms of the Development and Option Agreement, the Company is also responsible for the FTE funding obligations and reimbursements to Acuitas for certain development and material costs incurred by them, which is currently approximately $ 0.4 million per year. In March 2021, the Company exercised the first option under the Development and Option Agreement and entered into a non-exclusive license agreement with Acuitas (the “Acuitas License Agreement”) under which the Company was granted a non-exclusive, worldwide, sublicensable license under the Acuitas LNP Technology to research, develop, manufacture, and commercially exploit products consisting of the Company’s gene modulating therapeutics and Acuitas’s lipid nanoparticles. In connection with the option exercise, the Company paid Acuitas an option exercise fee of $ 1.5 million. Under the Acuitas License Agreement, the Company is required to pay Acuitas an annual license maintenance fee in the high six figures until the Company achieves a particular development milestone. Acuitas is entitled to receive potential clinical and regulatory milestone payments of up to $ 18.0 million in the aggregate. With respect to the sale of each licensed products, the Company is also obligated to pay Acuitas low single digit percentage royalties on net sales of the licensed products by the Company and its affiliates and sublicensees in a given country until the last to occur, in such country, of (i) the expiration or abandonment of all licensed patent rights covering the licensed product, (ii) expiration of any regulatory exclusivity for the licensed product, or (iii) ten years from the first commercial sale of the licensed product. During the three and nine months ended September 30, 2021 , the Company recorded an aggregate of $ 0.6 million and $ 2.6 million of research and development expenses, respectively, consisting of the option exercise fee, the costs of services performed by Acuitas, the material costs and the reimbursable costs incurred. The option exercise fee under the Development and Option Agreement was recorded as research and development expense upon the Company’s exercise of the first option. Additionally, the technology access fees, target reservation and maintenance fees, expenses associated with the FTE funding obligations and reimbursements for development and material costs incurred by Acuitas are recorded as research and development expense when incurred. The annual maintenance fee will be recorded as an expense on an annual basis based on the stated amount for the applicable year. Upon determination that a milestone payment is probable to occur, the amount due will be recorded as research and development expense. As the triggering of these milestone payments was not considered probable during the three and nine months ended September 30, 2021 , no expense has been recorded for these milestones during the periods. Lastly, the royalty payment is contingent upon sales of licensed products under the Acuitas License Agreement. As such, when such expenses are considered probable and estimable at the commencement of sales, the Company will accrue royalty expense for the amount the Company is obligated.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Redeemable Convertible Preferred Stock	10. Redeemable Convertible Preferred Stock During August 2017 and March 2021, the Company issued Series A, Series B and Series C Preferred Stock (collectively the “Preferred Stock”) to investors in private placements. Upon completion of the Company’s IPO, all of the Company’s then outstanding Preferred Stock was automatically converted into an aggregate of 34,678,733 shares of common stock. Series A Redeemable Convertible Preferred Stock During 2017 and 2019, the Company issued an aggregate of 56,775,232 shares of Series A Preferred Stock at a purchase price of $ 0.50 per share in exchange for cash proceeds of $ 25.5 million and the exchange of approximately $ 2.8 million in outstanding promissory notes, including $ 54 thousand in accrued interest. In connection with the issuance of Series A Preferred Stock during 2017 and 2019, the Company incurred less than $ 0.1 million of issuance costs. No additional shares of Series A Preferred Stock were issued after 2019. Series B Redeemable Convertible Preferred Stock On January 27, 2020, the Company issued 24,066,666 shares of Series B Preferred Stock at a purchase price of $ 1.50 per share for aggregate proceeds of $ 36.1 million. On June 2, 2020, the Company issued an additional 3,333,333 shares of Series B Preferred Stock at the purchase price of $ 1.50 per share for aggregate proceeds of $ 5.0 million. On August 3, 2020, the Company issued 5,000,000 shares of Series B Preferred Stock at the purchase price of $ 1.50 per share for aggregate proceeds of $ 7.5 million. No additional shares of Series B Preferred Stock were issued after August 2020. In connection with the issuance of Series B Preferred Stock in 2020, the Company incurred $ 83 thousand of issuance costs. Series C Redeemable Convertible Preferred Stock In March 2021, the Company issued an aggregate of 41,833,328 shares of Series C Preferred Stock, at a price of $ 3.00 per share, for gross proceeds of $ 125.5 million. The terms of the Series C Preferred Stock are substantially the same as the terms of the Series A and Series B Preferred Stock. No additional shares of Series C Preferred Stock were issued after March 2021. In connection with the issuance of Series C Preferred Stock in March 2021, the Company incurred $ 0.1 million of issuance costs. The Company further amended its Amended and Restated Certificate of Incorporation to increase the authorized preferred stock issuable from 107,125,232 shares to 132,858,564 shares. As of December 31, 2020, the Preferred Stock consisted of the following (in thousands, except for share data): December 31, 2020 Preferred Preferred Carrying Liquidation Common Series A Preferred Stock 57,125,232 56,775,232 $ 26,708 $ 25,500 15,028,741 Series B Preferred Stock 50,000,000 32,399,999 48,517 48,600 8,576,470 107,125,232 89,175,231 $ 75,225 $ 74,100 23,605,211

Preferred and Common Stock

Preferred and Common Stock	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Preferred and Common Stock	11. Preferred and Common Stock The Company’s board of directors and stockholders approved an amendment to the Company’s certificate of incorporation, which became effective on July 23, 2021. The amendment, among other things, effected a 1-for-3.777776 reverse stock split of the Company’s issued and outstanding common stock, a proportional adjustment to the conversion price for Series A, B and C Preferred Stock and to the exercise prices and number of shares of common stock underlying the outstanding stock options. All share, per share and additional paid in capital amounts for all periods presented in the accompanying unaudited condensed financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect the reverse stock split. In August 2021, the Company completed its IPO pursuant to which it issued and sold 8,300,976 shares of its common stock, including 900,976 shares pursuant to the partial exercise of the underwriters’ option to purchase additional shares, at a public offering price of $ 17.00 per share, for aggregate gross proceeds of $ 141.1 million. The Company received approximately $ 128.1 million in net proceeds after deducting underwriting discounts and commissions and other offering expenses payable by the Company. In connection with the completion of the IPO, the Company’s board of directors and stockholders approved the Company’s Amended and Restated Certificate of Incorporation to, among other things, provide for 200,000,000 authorized shares of common stock with a par value of $ 0.001 per share and 10,000,000 authorized shares of preferred stock with a par value of $ 0.001 per share. The holders of common stock are entitled to one vote for each share of common stock. Subject to the payment in full of all preferential dividends to which the holders of the preferred stock are entitled, the holders of common stock shall be entitled to receive dividends out of funds legally available. In the event of any voluntary or involuntary liquidation, dissolution, or winding up of the Company, after the payment or provision for payment of all debts and liabilities of the Company and all preferential amounts to which the holders of preferred stock are entitled with respect to the distribution of assets in liquidation, the holders of common stock shall be entitled to share ratably in the remaining assets of the Company available for distribution. As of September 30, 2021 , the Company has reserved an aggregate of 5,575,623 shares of common stock for the potential exercise of outstanding stock options under its equity incentive plans. Upon the effectiveness of the 2021 Incentive Award Plan (“2021 Plan”), the Company ceased granting awards under the 2017 Equity Incentive Plan (“2017 Plan”), and the 5,014,547 shares of common stock subject to outstanding stock options issued under the 2017 Plan may become available for future issuance under the 2021 Plan to the extent such stock options are forfeited.

Equity Incentive Plan

Equity Incentive Plan	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity Incentive Plan	12. Equity Incentive Plan 2017 Equity Incentive Plan In June 2017, the Company’s board of directors adopted the 2017 Plan, which provided for the grant of qualified incentive stock options and nonqualified stock options, restricted stock or other awards to the Company’s employees and non-employees for the issuance or purchase of shares of the Company’s common stock. As of September 30, 2021 , there were no shares available for future grants under the 2017 Plan and a total of 5,014,547 shares of the Company’s common stock were subject to outstanding stock options issued under the 2017 Plan. The 2017 Plan is administered by the Company’s board of directors or a committee thereof to the extent the Company’s board of directors has delegated its power or authority under the 2017 Plan. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, except that the exercise price per share of stock options may not be less than 100 % of the fair market value of the common stock on the date of grant. Stock options awarded under the 2017 Plan expire 10 years after the grant date unless the board of directors sets a shorter term. Incentive stock options and nonqualified stock options granted to employees and non-employees typically vest over four years . Certain stock options provide for accelerated vesting if there is a change in control, as defined in the 2017 Plan. 2021 Equity Incentive Plan The Company's board of directors adopted, and the Company’s stockholders approved the 2021 Plan in July 2021. The 2021 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards, and subsequent to the IPO, all equity-based awards are granted under the 2021 Plan. The Company has reserved 2,960,000 shares of its common stock for future issuance under the 2021 Plan. As of September 30, 2021 , there were 2,407,513 shares available for future grants under the 2021 Plan, and a total of 561,076 shares of the Company’s common stock were subject to outstanding stock options issued under the 2021 Plan. The Company recorded stock-based compensation expense as research and development and general and administrative expenses in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Research and development $ 438 $ 108 $ 713 $ 248 General and administrative 685 121 1,289 233 Total stock-based compensation expense $ 1,123 $ 229 $ 2,002 $ 481 Stock Options A summary of option activity under the Company’s equity incentive plans during the nine months ended September 30, 2021 was as follows: Number Weighted Weighted Aggregate Outstanding as of January 1, 2021 3,049,875 $ 1.10 8.86 $ 4,467 Granted 2,875,899 8.05 Exercised ( 241,312 ) 1.04 Forfeitures ( 108,839 ) 2.98 Outstanding at September 30, 2021 5,575,623 4.65 8.89 79,151 Vested and expected to vest as of September 30, 2021 5,575,623 4.65 8.89 79,151 Exercisable as of September 30, 2021 1,161,179 0.88 8.05 20,870 (1) 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 . The weighted-average grant date fair value per share of stock options granted during the nine months ended September 30, 2021 was $ 5.49 . The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2021 was $ 4.3 million. As of September 30, 2021 , there was $ 15.3 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of approximately 3.5 years. Restricted Stock In 2017, the Company issued 1,985,295 shares of restricted common stock to certain scientific founders, having a fair value of $ 0.8 million, and subject to vesting over a period of 4 years. If the holders of restricted common stock cease to have a service relationship with the Company prior to the vesting of such shares, the Company may reacquire any unvested shares of common stock held by these individuals for the original purchase price, and in certain instances for no consideration. The unvested shares of restricted common stock are not considered outstanding shares for accounting purposes until the shares vest. As the restricted stock was fully vested in 2020, there was no remaining unrecognized stock-based compensation expense related to restricted stock as of September 30, 2021 .

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	13. Net Loss per Share Attributable to Common Stockholders For periods in which the Company reports a net loss attributable to common stockholders, potentially dilutive securities have been excluded from the computation of diluted net loss per share as their effects would be anti-dilutive. Therefore, the weighted average number of shares of common stock outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Numerator: Net loss attributable to common stockholders $ ( 18,528 ) $ ( 8,241 ) $ ( 47,426 ) $ ( 19,982 ) Denominator: Weighted average number of common stock, basic and diluted 32,303,540 4,180,696 13,898,089 4,014,800 Net loss per common stock attributable to common stockholders, basic and diluted $ ( 0.57 ) $ ( 1.97 ) $ ( 3.41 ) $ ( 4.98 ) The Company excluded the following potential common stock, presented based on amounts outstanding at period end, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of September 30, 2021 2020 Redeemable convertible preferred stock — 23,605,211 Unvested restricted stock — 124,082 Outstanding options to purchase common stock 5,575,623 3,244,633 Warrants — 92,647 Total 5,575,623 27,066,573

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes During the three and nine months ended September 30, 2021 and 2020 , the Company recorded a full valuation allowance on federal and state deferred tax assets since management does not forecast the Company to be in a taxable position in the near future.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	15. Related Party Transactions The majority ownership of the Company is held by Flagship, in which it holds shares representing approximately 54 % of the Company’s outstanding voting stock as of September 30, 2021. Flagship provides management services (accounting, human resources, information technology, legal and consultation) to the Company. Flagship is also reimbursed for certain expenses, including insurance and benefits, partner and related fees, and software licenses incurred on the Company’s behalf. During both of the three months ended September 30, 2021 and 2020 , the Company incurred $ 0.2 million for management services fees and other reimbursable expenses. During both of the nine months ended September 30, 2021 and 2020 , the Company incurred $ 0.7 million for management services fees and other reimbursable expenses. These expenses are recorded as related party expense in the accompanying statements of operations and comprehensive loss. As of September 30, 2021 and December 31, 2020 , the Company did no t have any outstanding payments due to Flagship. In September 2020, the Company sublet the entire space of its 325 Vassar Street facility, approximately 19,404 square feet, to LARONDE, Inc. (“LARONDE”, formerly known as VL50, Inc.), which is an affiliate of Flagship. The sublease term will expire at the end of the Company’s lease agreement with the landlord in September 2024. The rental rate for the sublease arrangement is equal to the Company’s rental obligation per the agreement with BMR-325 Vassar Street LLC, reduced by the sublease income received from Cygnal Therapeutics, Inc. (“Cygnal”), approximating $ 1.3 million per year. The sublessee is obligated to pay all real estate taxes and costs related to the subleased premises, including cost of operations, maintenance, repair, replacement and property management. Under the sublease agreement, the Company received rental income, which was recorded as a reduction of rental expenses, of $ 0.5 million and $ 1.5 million during the three and nine months ended September 30, 2021 , respectively, and $ 0.2 million during both of the three and nine months ended September 30, 2020 . Such rental income was reflected as a reduction of related party expense in the accompanying statements of operations and comprehensive loss. There was no outstanding receivable due from LARONDE as of September 30, 2021 and December 31, 2020. In September 2019, the Company sublet approximately 1,445 square feet of its 325 Vassar Street facility to Cygnal, which is an affiliate of Flagship, for two years . The lease term can continue on a month-to-month basis until advanced notice is provided to the Company. Upon the lease term ended in September 2021, Cygnal elected to continue leasing the space until December 2022. The rental rate for the sublease arrangement is equal to the Company’s rental obligation per the agreement with BMR-325 Vassar Street LLC, approximating $ 0.1 million per year. The sublessee is obligated to pay all real estate taxes and costs related to the subleased premises, including cost of operations, maintenance, repair, replacement and property management. Under the sublease agreement, the Company received rental income of less than $ 0.1 million and $ 0.1 million during both of the three-month periods and nine-month periods ended September 30, 2021 and 2020 , respectively, which was recorded as a reduction of rental expenses. Such rental income was reflected as a reduction of related party expense in the accompanying statements of operations and comprehensive loss. There was no outstanding receivable due from Cygnal as of September 30, 2021 and December 31, 2020 .

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events Collaboration agreement On November 5, 2021, the Company entered into a collaboration agreement with PM (CF) Explorations, Inc., or PMCo, an affiliate of Flagship, under which PMCo was granted an exclusive license covering specified patent rights of the Company’s lipid nanoparticle technology to develop one or more therapeutic products to treat diseases related to the cystic fibrosis transmembrane conductance regulator gene, like cystic fibrosis. Under the terms of the agreement, the Company will perform certain research activities in accordance with the research plan, and PMCo will be solely responsible for, at its sole cost and expense, and will have sole discretion with respect to, developing, manufacturing, seeking regulatory approval for and commercializing licensed products. The Company will receive approximately $ 3.5 million in cost reimbursement over the course of the next two to three years to fund the related research and development activities. The research plan funding may be adjusted upon mutual written agreement from both parties. Additionally, in the event PMCo is acquired or sold, the Company is entitled to receive a portion of the proceeds of such transaction, subject to various reductions and other amounts payable in accordance with the agreement. One Charles Park lease agreement On November 4, 2021, the Company entered into a lease (the “Lease”) with ARE-MA Region No, 94, LLC (the “Landlord”), pursuant to which the Company agreed to lease an aggregate of approximately 89,246 rentable square feet of office and laboratory space located at One Charles Park, Cambridge, Massachusetts, 02142 (the “Premises”). The term of the Lease is estimated to begin on December 15, 2022 and end on January 1, 2037 , subject to certain extension rights. The term of the Lease will commence on the earlier of (i) the date the Landlord delivers the Premises to the Company with the tenant improvements substantially completed and (ii) the date in which the Landlord could have delivered the Premises to the Company, but for the Company’s delays, and last for a period of one hundred and eighty (180) months. Pursuant to the terms of the Lease, delivery of the Premises is conditioned upon Landlord acquiring fee title to the property or entering into a master lease to become the master lessee of the Property (the “Condition Precedent”) and, if it is not satisfied by January 26, 2022, the Lease may be terminated by the Landlord or Company by delivery of written notice to the other on or before February 2, 2022. The Lease provides that base rent for the Premises will be approximately $ 115.00 per square foot, subject to an annual upward adjustment of 3 % of the then current rental rate, starting on the first anniversary of the first payment of rent under the Lease, and other potential adjustments based on the Company’s utilization of certain tenant improvement allowances.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of the unaudited condensed financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these unaudited condensed financial statements include, but are not limited to, the selection of useful lives of property and equipment, the fair values of common stock, redeemable convertible preferred stock, and warrants prior to the IPO, the fair value of the success fee obligation, and stock-based compensation. Actual results could differ from these estimates. Changes in estimates are reflected in reported results in the period in which they become known.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ deficit as a reduction of proceeds generated as a result of the offering. Should a planned equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the unaudited condensed statement of operations. Upon closing of the IPO in August 2021, deferred offering costs were derecognized and recorded against the IPO proceeds as a reduction to additional paid-in capital. There were no deferred offering costs as of September 30, 2021 .
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight- line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. As the Company has elected to use the extended transition period for complying with new or revised accounting standards as available under the Jumpstart Our Business Startups Act (“JOBS Act”), the standard is effective for the Company beginning January 1, 2022, with early adoption permitted. The Company engaged an external third party to assist with the adoption of ASU 2016-02 and is currently assessing the impact it will have on its financial statements. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes (“ASU 2019-12”). ASU 2019-12 eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new guidance also simplifies aspects of the accounting for franchise taxes, enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. Adoption of the standard requires certain changes to be made prospectively and certain others to be made retrospectively. As the Company has elected to use the extended transition period for complying with new or revised accounting standards as available under the JOBS Act, the standard is effective for the Company beginning January 1, 2022, with early adoption permitted. The Company does not expect ASU 2019-12 to have a material effect on the Company’s financial statements.

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Prepaid Expense And Other Assets Current [Abstract]
Summary of Prepaid Expenses and other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): September 30, December 31, 2021 2020 Prepaid software $ 127 $ 78 Prepaid research and development 1,355 653 Prepaid other 507 120 Prepaid insurance 2,128 8 Other receivables 367 193 Prepaid expenses and other current assets $ 4,484 $ 1,052

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consists of the following (in thousands): September 30, December 31, 2021 2020 Leasehold improvements $ 1,378 $ 1,378 Lab equipment 4,532 3,444 Furniture and fixtures 1,047 985 Computer equipment 129 129 Construction in process 165 16 Total property and equipment 7,251 5,952 Less accumulated depreciation ( 3,466 ) ( 2,470 ) Property and equipment, net $ 3,785 $ 3,482

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Accrued Liabilities Current [Abstract]
Summary of Accrued Expenses	Accrued expenses consist of the following (in thousands): September 30, December 31, 2021 2020 Employee related expenses $ 1,771 $ 1,124 Research costs 5,082 1,724 Professional and consulting fees 1,403 278 Interest 58 44 Other 397 107 Total $ 8,711 $ 3,277

Term Loan (Tables)

Term Loan (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Summary of Outstanding Term Loan Balance	The Company’s outstanding term loan balance was comprised of the following (in thousands): September 30, December 31, 2021 2020 Principal $ 12,000 $ 12,000 Unamortized debt discount ( 15 ) ( 268 ) Net carrying amount $ 11,985 $ 11,732
Summary of Interest Expense Recognized Related to Term loan	The following table sets forth total interest expense recognized related to the term loan (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Contractual interest expense $ 184 $ 184 $ 546 $ 548 Amortization of debt issuance costs and debt discount 179 12 253 36 Total interest expense $ 363 $ 196 $ 799 $ 584
Schedule of Principal Amount in Connection With Term Loan	The Company is required to repay the following principal amounts in connection with its term loan (in thousands): 2021 (remaining 3 months) $ — 2022 6,000 2023 6,000 $ 12,000

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Fair Value of Financial Instruments	As of December 31, 2020, the fair value of the Company’s financial instruments is summarized in the table below (in thousands): December 31, 2020 Level 1 Level 2 Level 3 Total Financial Liabilities Warrant liability $ — $ — $ 124 $ 124 Success fee obligation — — 194 194 Total $ — $ — $ 318 $ 318
Summary of Assumptions Used in Black-Scholes Option-Pricing Model for Warrants	The assumptions used in the Black-Scholes option-pricing model for the warrants were as follows: August 3, December 31, 2021 2020 Expected volatility 77.03 - 77.62 % 73.09 - 77.79 % Risk-free interest rate 0.95 - 1.40 % 0.58 - 0.65 % Expected dividend yield 0.00 % 0.00 % Expected term (in years) 6.6 - 6.9 7.2 - 7.9
Summary of Fair Values of Warrant Liability and Success Fee Obligation	The following table provides a roll-forward of the fair values of the Company’s warrant liability and the success fee obligation for which fair value is determined by Level 3 inputs (in thousands): Warrant Success Fair value at January 1, 2021 $ 124 $ 194 Change in fair value 1,310 6 Settlement of success fee obligation — ( 200 ) Reclassification to additional paid-in-capital in connection with IPO ( 1,434 ) — Fair value at September 30, 2021 $ — $ — Warrant Success Fair value at January 1, 2020 $ 127 $ — Change in fair value ( 5 ) — Fair value at September 30, 2020 $ 122 $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments	As of September 30, 2021, the future minimum lease payments for the Company’s facility operating leases were as follows (in thousands): 2021 (remaining 3 months) $ 839 2022 2,618 2023 1,563 2024 1,205 Total minimum lease payments $ 6,225

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Summary of Preferred Stock	As of December 31, 2020, the Preferred Stock consisted of the following (in thousands, except for share data): December 31, 2020 Preferred Preferred Carrying Liquidation Common Series A Preferred Stock 57,125,232 56,775,232 $ 26,708 $ 25,500 15,028,741 Series B Preferred Stock 50,000,000 32,399,999 48,517 48,600 8,576,470 107,125,232 89,175,231 $ 75,225 $ 74,100 23,605,211

Equity Incentive Plan (Tables)

Equity Incentive Plan (Tables)	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock-based Compensation Expense	The Company recorded stock-based compensation expense as research and development and general and administrative expenses in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Research and development $ 438 $ 108 $ 713 $ 248 General and administrative 685 121 1,289 233 Total stock-based compensation expense $ 1,123 $ 229 $ 2,002 $ 481
Summary of Option Activity	A summary of option activity under the Company’s equity incentive plans during the nine months ended September 30, 2021 was as follows: Number Weighted Weighted Aggregate Outstanding as of January 1, 2021 3,049,875 $ 1.10 8.86 $ 4,467 Granted 2,875,899 8.05 Exercised ( 241,312 ) 1.04 Forfeitures ( 108,839 ) 2.98 Outstanding at September 30, 2021 5,575,623 4.65 8.89 79,151 Vested and expected to vest as of September 30, 2021 5,575,623 4.65 8.89 79,151 Exercisable as of September 30, 2021 1,161,179 0.88 8.05 20,870 (1) 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 .

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]
Summary of the Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Numerator: Net loss attributable to common stockholders $ ( 18,528 ) $ ( 8,241 ) $ ( 47,426 ) $ ( 19,982 ) Denominator: Weighted average number of common stock, basic and diluted 32,303,540 4,180,696 13,898,089 4,014,800 Net loss per common stock attributable to common stockholders, basic and diluted $ ( 0.57 ) $ ( 1.97 ) $ ( 3.41 ) $ ( 4.98 )
Schedule of Anti-Dilutive Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential common stock, presented based on amounts outstanding at period end, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of September 30, 2021 2020 Redeemable convertible preferred stock — 23,605,211 Unvested restricted stock — 124,082 Outstanding options to purchase common stock 5,575,623 3,244,633 Warrants — 92,647 Total 5,575,623 27,066,573

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
	Aug. 31, 2021	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued		47,768,565	4,465,351
Gross proceeds from IPO		$ 131,238
Conversion of preferred stock to common stock			23,605,211
Cash and cash equivalents		$ 234,275	$ 22,951	$ 31,728
IPO
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued	8,300,976
Offering price per share	$ 17
Gross proceeds from IPO	$ 141,100
Net proceeds	$ 128,100
Underwriters’ Option to Purchase Additional Shares
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued	900,976

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details)	Sep. 30, 2021USD ($)
Accounting Policies [Abstract]
Deferred offering costs	$ 0

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid software	$ 127	$ 78
Prepaid research and development	1,355	653
Prepaid other	507	120
Prepaid insurance	2,128	8
Other receivables	367	193
Prepaid expenses and other current assets	$ 4,484	$ 1,052

Property and Equipment, Net - S

Property and Equipment, Net - Summary 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	$ 7,251	$ 5,952
Less accumulated depreciation	(3,466)	(2,470)
Property and equipment, net	3,785	3,482
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	1,378	1,378
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	4,532	3,444
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	1,047	985
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	129	129
Construction in Process
Property Plant And Equipment [Line Items]
Total property and equipment	$ 165	$ 16

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property Plant And Equipment [Abstract]
Depreciation	$ 400	$ 300	$ 1,003	$ 715

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accrued Liabilities Current [Abstract]
Employee related expenses	$ 1,771	$ 1,124
Research costs	5,082	1,724
Professional and consulting fees	1,403	278
Interest	58	44
Other	397	107
Total	$ 8,711	$ 3,277

Term Loan - Additional Informat

Term Loan - Additional Information (Details)	Dec. 30, 2020USD ($)	Mar. 09, 2018USD ($)$ / sharesshares	Aug. 31, 2021shares	Jan. 31, 2020	Sep. 30, 2019USD ($)$ / sharesshares	Sep. 30, 2021USD ($)	Dec. 31, 2020USD ($)	Aug. 03, 2021shares	Sep. 29, 2019USD ($)	Aug. 31, 2018USD ($)
Debt [Line Items]
Aggregate principal amount						$ 12,000,000	$ 12,000,000
Unamortized debt discount						15,000	268,000
Long-term debt, current portion						4,500,000	3,000,000
Long-term debt						7,500,000	9,000,000
Accrued interest on term loan						$ 58,000	$ 44,000
Maximum
Debt [Line Items]
Effective interest rate on the liability component						7.51%
Minimum
Debt [Line Items]
Effective interest rate on the liability component						5.53%
Pacific Western Bank
Debt [Line Items]
Maturity date						Dec. 31, 2023
Principal repayment date						Dec. 31, 2021
Pacific Western Bank | Minimum
Debt [Line Items]
Cash proceeds received						$ 50,000,000
Loan Agreement | Pacific Western Bank
Debt [Line Items]
Tranche I funded date as Closing date						Mar. 9, 2018
Tranche II funded date						no later than 18 months from the Closing Date
Line of credit facility, Floating annual interest rate						0.75%
Line of credit facility, Interest rate during period						5.00%
Loan Agreement | Pacific Western Bank | IPO
Debt [Line Items]
Success fee payable						$ 200,000
Loan Agreement | Pacific Western Bank | Series A Redeemable Convertible Preferred Stock
Debt [Line Items]
Shares, Issued | shares		87,500
Shares issued, Price per share | $ / shares		$ 0.5
Warrant exercisable period		10 years
Loan Agreement | Pacific Western Bank | Series A Redeemable Convertible Preferred Stock | Maximum
Debt [Line Items]
Warrant exercisable for number of shares | shares		200,000
Loan Agreement | Pacific Western Bank | Tranche I and Tranche II
Debt [Line Items]
Maximum borrowing capacity		$ 8,000,000
Borrowed loan									$ 8,000,000
Loan Agreement | Pacific Western Bank | Tranche I
Debt [Line Items]
Borrowed loan		$ 3,500,000
Loan Agreement | Pacific Western Bank | Tranche II
Debt [Line Items]
Borrowed loan										$ 4,500,000
Loan Agreement | Pacific Western Bank | Tranche II | Series A Redeemable Convertible Preferred Stock
Debt [Line Items]
Warrant exercisable multiplication rate		0.0125
First Amendment | Pacific Western Bank
Debt [Line Items]
Warrants exercisable, expiration date					2028-03
Number of shares withheld to pay exercise price of warrants | shares			10,454
First Amendment | Pacific Western Bank | IPO
Debt [Line Items]
Unamortized debt discount						$ 100,000
First Amendment | Pacific Western Bank | Series A Redeemable Convertible Preferred Stock
Debt [Line Items]
Shares, Issued | shares					350,000
Shares issued, Price per share | $ / shares					$ 0.50
First Amendment | Pacific Western Bank | Common Stock
Debt [Line Items]
Warrant exercisable for number of shares | shares			82,193
First Amendment | Pacific Western Bank | Common Stock | IPO
Debt [Line Items]
Shares, Issued | shares								92,647
First Amendment | Pacific Western Bank | Tranche I and Tranche II
Debt [Line Items]
Debt extinguished					$ 8,000,000
First Amendment | Pacific Western Bank | Tranche III
Debt [Line Items]
Line of credit facility, Floating annual interest rate					0.75%
Line of credit facility, Interest rate during period					6.00%
Aggregate principal amount					$ 12,000,000
Remaining cash proceeds					$ 4,000,000
Maturity date				Jun. 9, 2023	Mar. 9, 2022
Debt issuance costs					$ 15,000
Third Amendment | Pacific Western Bank
Debt [Line Items]
Line of credit facility, Floating annual interest rate	0.75%
Line of credit facility, Interest rate during period	6.00%
Maturity date	Jun. 30, 2023
Debt issuance costs	$ 15,000

Term Loan - Summary of Outstand

Term Loan - Summary of Outstanding Term Loan Balance (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Principal	$ 12,000	$ 12,000
Unamortized debt discount	(15)	(268)
Net carrying amount	$ 11,985	$ 11,732

Term Loan - Summary of Interest

Term Loan - Summary of Interest Expense Recognized Related to Term loan (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Disclosure [Abstract]
Contractual interest expense	$ 184	$ 184	$ 546	$ 548
Amortization of debt issuance costs and debt discount	179	12	253	36
Total interest expense	$ 363	$ 196	$ 799	$ 584

Term Loan - Schedule of Princip

Term Loan - Schedule of Principal Amount in Connection With Term Loan (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
2021 (remaining 3 months)	$ 0
2022	6,000
2023	6,000
Long-term Debt	$ 12,000	$ 12,000

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Summary of Fair Value of Financial Instruments (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Financial Liabilities
Total financial liabilities		$ 318,000
Warrant Liability
Financial Liabilities
Total financial liabilities	$ 0	124,000
Success Fee Obligation
Financial Liabilities
Total financial liabilities	$ 0	194,000
Level 3
Financial Liabilities
Total financial liabilities		318,000
Level 3 | Warrant Liability
Financial Liabilities
Total financial liabilities		124,000
Level 3 | Success Fee Obligation
Financial Liabilities
Total financial liabilities		$ 194,000

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Aug. 03, 2021	Dec. 31, 2020USD ($)
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Financial liabilities						$ 318,000
Description of warrant liability			As of December 31, 2020, the warrant liability was recorded as other liabilities on the balance sheet as it was deemed more probable than not by the Company to be settled in longer than one year.
Change in fair value of warrant liability	$ 970,000	$ (1,000)	$ 1,310,000	$ (5,000)
Description of fair value of success fee obligation			As a result of the completion of the IPO in August 2021, the Company remeasured the fair value of the success fee obligation to the full payment amount.
Warrant Liability
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Financial liabilities	0		$ 0			124,000
Success Fee Obligation
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Financial liabilities	$ 0		$ 0			$ 194,000
Expected Dividend Yield
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input	0		0		0	0

Fair Value of Financial Instr_5

Fair Value of Financial Instruments - Summary of Assumptions Used in Black-Scholes Option-Pricing Model for Warrants (Details)	Sep. 30, 2021	Aug. 03, 2021yr	Dec. 31, 2020yr
Expected Volatility | Minimum
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input		77.03	73.09
Expected Volatility | Maximum
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input		77.62	77.79
Risk-free Interest Rate | Minimum
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input		0.95	0.58
Risk-free Interest Rate | Maximum
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input		1.40	0.65
Expected Dividend Yield
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input	0	0	0
Expected Term (in years) | Minimum
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input		6.6	7.2
Expected Term (in years) | Maximum
Fair Value Liabilities Measured On Recurring Basis [Line Items]
Warrants, measurement input		6.9	7.9

Fair Value of Financial Instr_6

Fair Value of Financial Instruments - Summary of Fair Values of Warrant Liability and Success Fee Obligation (Details) - Level 3 - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Warrant Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 124	$ 127
Change in fair value	1,310	(5)
Reclassification to additional paid-in-capital in connection with IPO	(1,434)
Ending balance	0	$ 122
Success Fee Obligation
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	194
Change in fair value	6
Settlement of success fee obligation	(200)
Ending balance	$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)ft²Option	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)Sublease	Dec. 31, 2019Sublease	Dec. 31, 2018USD ($)
Leases [Line Items]
Lease expiration date			Sep. 30, 2024
Landlord-funded leasehold improvements							$ 1,100,000
Rent expenses incurred	$ 600,000	$ 400,000	$ 1,900,000	$ 400,000
Rent expense	800,000	$ 700,000	$ 2,500,000	$ 1,400,000
Senda Biosciences, Inc
Leases [Line Items]
Lessee operating lease, description			share one-third of Senda’s 69,867 square feet of leased space at 20 Acorn Park Drive, Cambridge, Massachusetts.
Number of options to extend terms | Option			2
Term of lease arrangement period			24 months
Lease arrangement description			The Arrangement commenced on August 1, 2020 and continues through July 31, 2022 with two options to extend the term of the Arrangement for a period of 24 months each.
Area of leased space | ft²			69,867
Accrued rent	$ 0		$ 0		$ 0
LARONDE, Inc.
Leases [Line Items]
Number of sublease agreements with related parties | Sublease					2
Cygnal
Leases [Line Items]
Number of sublease agreements with related parties | Sublease						2

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments (Details) $ in Thousands	Sep. 30, 2021USD ($)
Leases [Abstract]
2021 (remaining 3 months)	$ 839
2022	2,618
2023	1,563
2024	1,205
Total minimum lease payments	$ 6,225

License Agreements - Additional

License Agreements - Additional Information (Details)	1 Months Ended			3 Months Ended		9 Months Ended
	Mar. 31, 2021USD ($)	Oct. 31, 2020USD ($)	May 31, 2019USD ($)Product	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
License Agreements [Line Items]
Milestone payments					$ 0		$ 0
License maintenances fees and reimbursable patent costs						$ 100,000	100,000
Outstanding payment due to WIBR				$ 0		0		$ 0
Research and development				12,289,000	5,505,000	$ 33,222,000	$ 13,921,000
Maximum
License Agreements [Line Items]
Reimbursable patent costs recognized				100,000	$ 100,000
WIBR Exclusive License Agreement
License Agreements [Line Items]
Number of licensed products | Product			3
Percentage of non-royalty payments description						This percentage ranges from zero to low double-digits and will be based upon the stage of development of the licensed product at the time such sublicense is executed.
WIBR Exclusive License Agreement | Maximum
License Agreements [Line Items]
Nonrefundable upfront fee paid.			$ 100,000
Annual license maintenance fees			100,000
Milestone payments			$ 1,700,000	100,000
WIBR Co-Exclusive License Agreement
License Agreements [Line Items]
Number of licensed products | Product			3
WIBR Co-Exclusive License Agreement | Maximum
License Agreements [Line Items]
Nonrefundable upfront fee paid.			$ 100,000
Annual license maintenance fees			100,000
Milestone payments			1,900,000
Annual fees for each sublicense agreement			$ 100,000
Development and Option Agreement
License Agreements [Line Items]
Reimbursements for development and material costs incurred by Acuitas		$ 400,000
Development and Option Agreement | First Non-exclusive License
License Agreements [Line Items]
Annual technology access fee		1,500,000
Development and Option Agreement | Second Non-exclusive License
License Agreements [Line Items]
Annual technology access fee		$ 1,750,000
Acuitas License Agreement
License Agreements [Line Items]
Milestone payments				0		$ 0
Option exercise fee	$ 1,500,000
Potential clinical and regulatory milestone payments aggregate amount entitled to receive Acuitas	$ 18,000,000
Research and development				$ 600,000		$ 2,600,000

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock - Additional Information (Details) - USD ($)	Aug. 03, 2021	Aug. 03, 2020	Jun. 02, 2020	Jan. 27, 2020	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017	Sep. 30, 2021	Jun. 30, 2021	Apr. 01, 2021	Sep. 01, 2020	Jan. 01, 2020
Class Of Stock [Line Items]
Number of preferred stock converted into common stock	34,678,733
Redeemable convertible preferred stock, shares issued						89,175,231
Redeemable convertible preferred stock, shares authorized					107,125,232	107,125,232				132,858,564
Series A Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares issued						56,775,232	56,775,232		0				0
Purchase price per share							$ 0.50	$ 0.50
Cash proceeds from issuance of convertible preferred stock							$ 25,500,000
Proceeds from outstanding promissory notes							2,800,000	$ 2,800,000
Accrued interest on outstanding promissory notes							54,000	$ 54,000
Redeemable convertible preferred stock, shares authorized						57,125,232			0
Series A Redeemable Convertible Preferred Stock | Maximum
Class Of Stock [Line Items]
Stock issuance costs							$ 100,000
Series B Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares issued		5,000,000	3,333,333	24,066,666		32,399,999			0			0
Purchase price per share		$ 1.50	$ 1.50	$ 1.50
Cash proceeds from issuance of convertible preferred stock		$ 7,500,000	$ 5,000,000	$ 36,100,000
Stock issuance costs						$ 83,000
Redeemable convertible preferred stock, shares authorized						50,000,000			0
Series C Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Redeemable convertible preferred stock, shares issued					41,833,328	0			0		0
Purchase price per share					$ 3
Cash proceeds from issuance of convertible preferred stock					$ 125,500,000
Stock issuance costs					$ 100,000
Redeemable convertible preferred stock, shares authorized						0			0

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Apr. 01, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Sep. 01, 2020	Aug. 03, 2020	Jun. 30, 2020	Jun. 02, 2020	Mar. 31, 2020	Jan. 27, 2020	Jan. 01, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Temporary Equity Shares Authorized		132,858,564		107,125,232	107,125,232
Temporary Equity Shares Issued					89,175,231
Temporary Equity Shares Outstanding					89,175,231
Temporary Equity Carrying Amount Attributable To Parent					$ 75,225
Temporary Equity Liquidation Preference					$ 74,100
Convertible Preferred Stock Shares Issued Upon Conversion					23,605,211
Series A Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Temporary Equity Shares Authorized	0				57,125,232
Temporary Equity Shares Issued	0				56,775,232								0	56,775,232
Temporary Equity Shares Outstanding	0	56,775,232		56,775,232	56,775,232	56,775,232			56,775,232		56,775,232			56,775,232
Temporary Equity Carrying Amount Attributable To Parent	$ 0	$ 26,708		$ 26,708	$ 26,708	$ 26,708			$ 26,708		$ 26,708			$ 26,708
Temporary Equity Liquidation Preference					$ 25,500
Convertible Preferred Stock Shares Issued Upon Conversion					15,028,741
Series B Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Temporary Equity Shares Authorized	0				50,000,000
Temporary Equity Shares Issued	0				32,399,999		0	5,000,000		3,333,333		24,066,666
Temporary Equity Shares Outstanding	0	32,399,999		32,399,999	32,399,999	32,399,999			27,399,999		24,066,666
Temporary Equity Carrying Amount Attributable To Parent	$ 0	$ 48,517		$ 48,517	$ 48,517	$ 48,517			$ 41,027		$ 36,038
Temporary Equity Liquidation Preference					$ 48,600
Convertible Preferred Stock Shares Issued Upon Conversion					8,576,470
Series C Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Temporary Equity Shares Authorized	0				0
Temporary Equity Shares Issued	0		0	41,833,328	0
Temporary Equity Shares Outstanding	0	41,833,328		41,833,328	0
Temporary Equity Carrying Amount Attributable To Parent	$ 0	$ 125,368		$ 125,368	$ 0

Preferred and Common Stock (Add

Preferred and Common Stock (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Aug. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock issued and sold		47,768,565	4,465,351
Gross proceeds from IPO		$ 131,238
Shares of common stock authorized		200,000,000	137,700,000
Common stock, par value		$ 0.001	$ 0.001
Shares of preferred stock authorized		10,000,000	0
Preferred stock, par value		$ 0.001	$ 0.001
IPO
Class Of Stock [Line Items]
Common stock issued and sold	8,300,976
Offering price per share	$ 17
Gross proceeds from IPO	$ 141,100
Net proceeds	$ 128,100
Underwriters’ Option to Purchase Additional Shares
Class Of Stock [Line Items]
Common stock issued and sold	900,976
2017 Plan
Class Of Stock [Line Items]
Common stock subject to outstanding stock options issued	5,014,547
2017 and 2021 Plans
Class Of Stock [Line Items]
Common stock subject to outstanding stock options issued		5,575,623
Amended and Restated Certificate of Incorporation
Class Of Stock [Line Items]
Reverse stock split description		1-for-3.777776
Shares of common stock authorized	200,000,000
Common stock, par value	$ 0.001
Shares of preferred stock authorized	10,000,000
Preferred stock, par value	$ 0.001

Equity Incentive Plan - Additio

Equity Incentive Plan - Additional Information (Details) - USD ($)	1 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2017	Sep. 30, 2021	Dec. 31, 2017	Jul. 31, 2021
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for future grant		0
Common stock subject to outstanding stock options issued		5,014,547
2017 Equity Incentive Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period	10 years
Vesting period	4 years
2017 Equity Incentive Plan | Stock Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of common stock percentage	100.00%
2017 Equity Incentive Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			4 years
Restricted common stock issued			1,985,295
Fair value of restricted common stock			$ 800,000
Unrecognized stock-based compensation expense related to restricted stock		$ 0
2021 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for future grant		2,407,513
Common stock subject to outstanding stock options issued		561,076
Common stock reserved for future issuance				2,960,000
2017 and 2021 Equity Incentive Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share of stock options granted		$ 5.49
Intrinsic value of stock options exercised		$ 4,300,000
Unrecognized compensation cost related to unvested stock options		$ 15,300,000
Unrecognized compensation cost, weighted-average period of recognition		3 years 6 months

Equity Incentive Plan - Summary

Equity Incentive Plan - 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
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,123	$ 229	$ 2,002	$ 481
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	438	108	713	248
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 685	$ 121	$ 1,289	$ 233

Equity Incentive Plan - Summa_2

Equity Incentive Plan - Summary of Option Activity (Details) - 2017 and 2021 Equity Incentive Plan $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of options Outstanding, Beginning balance | shares	3,049,875
Number of options, Granted | shares	2,875,899
Number of options, Exercised | shares	(241,312)
Number of options, Forfeitures | shares	(108,839)
Number of options Outstanding, Ending balance | shares	5,575,623	3,049,875
Number of options, Vested and expected to vest | shares	5,575,623
Number of options, Exercisable | shares	1,161,179
Weighted average exercise price Outstanding, Beginning balance | $ / shares	$ 1.10
Weighted average exercise price, Granted | $ / shares	8.05
Weighted average exercise price, Exercised | $ / shares	1.04
Weighted average exercise price, Forfeitures | $ / shares	2.98
Weighted average exercise price Outstanding, Ending balance | $ / shares	4.65	$ 1.10
Weighted average exercise price, Vested and expected to vest | $ / shares	4.65
Weighted average exercise price, Exercisable | $ / shares	$ 0.88
Weighted average remaining contractual life (years), Outstanding	8 years 10 months 20 days	8 years 10 months 9 days
Weighted average remaining contractual life (years), Vested and expected to vest	8 years 10 months 20 days
Weighted average remaining contractual life (years), Exercisable	8 years 18 days
Aggregate intrinsic value, Outstanding | $	$ 79,151	$ 4,467
Aggregate intrinsic value, Vested and expected to vest | $	79,151
Aggregate intrinsic value, Exercisable | $	$ 20,870

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Summary of the Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net loss attributable to common stockholders	$ (18,528)	$ (15,410)	$ (13,488)	$ (8,241)	$ (6,323)	$ (5,418)	$ (47,426)	$ (19,982)
Denominator:
Weighted average number of common stock, basic and diluted	32,303,540			4,180,696			13,898,089	4,014,800
Net loss per common stock attributable to common stockholders, basic and diluted	$ (0.57)			$ (1.97)			$ (3.41)	$ (4.98)

Net Loss Per Share Attributab_4

Net Loss Per Share Attributable to Common Stockholders - Schedule of Anti-Dilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	5,575,623	27,066,573
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		23,605,211
Unvested Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		124,082
Outstanding Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	5,575,623	3,244,633
Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		92,647

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)ft²	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)ft²	Dec. 31, 2020USD ($)	Sep. 30, 2019ft²
LARONDE, Inc.
Related Party Transaction [Line Items]
Area of real estate property | ft²		19,404		19,404
Sublease income			$ 1,300,000
Outstanding receivable	$ 0		0		$ 0
Cygnal
Related Party Transaction [Line Items]
Area of real estate property | ft²						1,445
Sublease income			100,000
Outstanding receivable	0		$ 0		0
Sublease term			2 years
Related Party Expense | LARONDE, Inc.
Related Party Transaction [Line Items]
Sublease rental income received	500,000	$ 200,000	$ 1,500,000	$ 200,000
Related Party Expense | Cygnal | Maximum
Related Party Transaction [Line Items]
Sublease rental income received	100,000	100,000	100,000	100,000
Flagship
Related Party Transaction [Line Items]
Outstanding payments	0		0		$ 0
Flagship | Related Party Expense | Management Services Fees and Other Reimbursements
Related Party Transaction [Line Items]
Related party expenses for management service	$ 200,000	$ 200,000	$ 700,000	$ 700,000
Omega Therapeutics, Inc. | Flagship
Related Party Transaction [Line Items]
Ownership percentage	54.00%		54.00%

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Subsequent Event $ in Millions	Nov. 05, 2021USD ($)	Nov. 04, 2021ft²$ / ft²
Collaboration Agreement
Subsequent Event [Line Items]
Amount of cost reimbursement receivable to fund research and development activities | $	$ 3.5
One Charles Park Lease Agreement
Subsequent Event [Line Items]
Rentable square feet of office and laboratory | ft²		89,246
Lessee operating lease, description		The term of the Lease is estimated to begin on December 15, 2022 and end on January 1, 2037, subject to certain extension rights.
Lease term beginning date		Dec. 15, 2022
Lease term ending date		Jan. 1, 2037
Base rent per square foot | $ / ft²		115
Base rent annual upward adjustment percentage		3.00%