Cartesian Therapeutics (RNAC) 10-Q 9 Nov 21 2021 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2021	Nov. 05, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	001-37798
Entity Registrant Name	Selecta Biosciences, Inc
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1622110
Entity Address, Address Line One	65 Grove Street,
Entity Address, City or Town	Watertown,
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02472
City Area Code	617
Local Phone Number	923-1400
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	SELB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Smaller Reporting Company	true
Entity Emerging Growth Company	true
Entity Extended Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		116,648,919
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001453687
Current Fiscal Year End Date	--12-31

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 114,645	$ 138,685
Marketable securities	24,018	0
Accounts receivable	7,324	7,224
Prepaid expenses and other current assets	5,781	5,434
Total current assets	151,768	151,343
Property and equipment, net	1,807	1,395
Right-of-use asset, net	10,117	10,948
Long-term restricted cash	1,379	1,379
Investments	2,000	0
Other assets	91	370
Total assets	167,162	165,435
Current liabilities:
Accounts payable	1,593	443
Accrued expenses	10,742	8,146
Loan payable	4,125	0
Lease liability	1,013	908
Income taxes payable	15,828	0
Deferred revenue	62,315	72,050
Total current liabilities	95,616	81,547
Non-current liabilities:
Loan payable, net of current portion	21,304	24,793
Lease liability	8,873	9,647
Deferred revenue	20,057	38,746
Warrant liabilities	40,043	28,708
Total liabilities	185,893	183,441
Commitments and contingencies
Stockholders’ (deficit) equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at September 30, 2021 and December 31, 2020	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 115,443,500 and 108,071,249 shares issued and outstanding as of September 30, 2021 and December 31, 2020, respectively	12	11
Additional paid-in capital	428,371	391,175
Accumulated deficit	(442,555)	(404,629)
Accumulated other comprehensive loss	(4,559)	(4,563)
Total stockholders’ (deficit) equity	(18,731)	(18,006)
Total liabilities and stockholders’ (deficit) equity	$ 167,162	$ 165,435

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	115,443,500	108,071,249
Common stock, shares outstanding (in shares)	115,443,500	108,071,249

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 24,427	$ 4,646	$ 55,140	$ 4,646
Operating expenses:
Research and development	20,951	13,960	48,418	39,414
General and administrative	5,445	4,420	15,397	14,155
Total operating expenses	26,396	18,380	63,815	53,569
Operating loss	(1,969)	(13,734)	(8,675)	(48,923)
Investment income	11	4	35	257
Loss on extinguishment of debt	0	(461)	0	(461)
Foreign currency transaction, net	2	43	(5)	83
Interest expense	(711)	(365)	(2,133)	(843)
Change in fair value of warrant liabilities	592	4,779	(11,335)	(3,606)
Other income, net	9	5	15	63
Loss before income taxes	(2,066)	(9,729)	(22,098)	(53,430)
Income tax expense	(15,828)	0	(15,828)	0
Net loss	(17,894)	(9,729)	(37,926)	(53,430)
Other comprehensive income (loss):
Foreign currency translation adjustment	(1)	(32)	5	(61)
Unrealized (losses) on marketable securities	(1)	0	(1)	0
Total comprehensive loss	$ (17,896)	$ (9,761)	$ (37,922)	$ (53,491)
Net loss per share:
Basic (in dollars per share)	$ (0.16)	$ (0.09)	$ (0.34)	$ (0.54)
Diluted (in dollars per share)	$ (0.16)	$ (0.09)	$ (0.34)	$ (0.54)
Weighted average common shares outstanding:
Basic (in shares)	115,169,949	105,325,788	113,161,622	98,968,359
Diluted (in shares)	115,169,949	105,325,788	113,161,622	98,968,359

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders’ (Deficit) Equity - USD ($) $ in Thousands	Total	Private Placement	Common stock	Common stockPrivate Placement	Additional paid-in capital	Additional paid-in capitalPrivate Placement	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2019			86,325,547
Beginning balance at Dec. 31, 2019	$ 8,397		$ 9		$ 348,664		$ (335,753)	$ (4,523)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)			78,583
Issuance of common stock under Employee Stock Purchase Plan	114				114
Issuance of common stock upon exercise of options (in shares)			5,128
Issuance of common stock upon exercise of options	3				3
Issuance of vested restricted stock units (in shares)			10,937
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)			598,977
Issuance of common stock through at-the-market offering, net	1,141				1,141
Other financing fees	(147)				(147)
Stock-based compensation expense	1,409				1,409
Currency translation adjustment	(60)							(60)
Net income (loss)	(19,620)						(19,620)
Ending balance (in shares) at Mar. 31, 2020			87,019,172
Ending balance at Mar. 31, 2020	(8,763)		$ 9		351,184		(355,373)	(4,583)
Beginning balance (in shares) at Dec. 31, 2019			86,325,547
Beginning balance at Dec. 31, 2019	8,397		$ 9		348,664		(335,753)	(4,523)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	(61)
Unrealized gains (losses) on marketable securities	0
Net income (loss)	(53,430)
Ending balance (in shares) at Sep. 30, 2020			107,235,976
Ending balance at Sep. 30, 2020	(7,382)		$ 11		386,374		(389,183)	(4,584)
Beginning balance (in shares) at Mar. 31, 2020			87,019,172
Beginning balance at Mar. 31, 2020	(8,763)		$ 9		351,184		(355,373)	(4,583)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)			37,500
Issuance of common stock upon exercise of options	98				98
Issuance of vested restricted stock units (in shares)			10,938
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)			470,509
Issuance of common stock through at-the-market offering, net	967				967
Issuance of common stock upon exercise of pre-funded warrants (in shares)			8,342,128
Issuance of common stock upon exercise of pre-funded warrants	1		$ 1
Issuance of common stock upon exercise of common warrants (in shares)			4,967,563
Issuance of common stock upon exercise of common warrants	17,214				17,214
Stock-based compensation expense	1,481				1,481
Currency translation adjustment	31							31
Net income (loss)	(24,081)						(24,081)
Ending balance (in shares) at Jun. 30, 2020			100,847,810
Ending balance at Jun. 30, 2020	(13,052)		$ 10		370,944		(379,454)	(4,552)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)			31,629
Issuance of common stock under Employee Stock Purchase Plan	70				70
Issuance of vested restricted stock units (in shares)			60,937
Issuance of vested restricted stock units	0
Issuance of common stock, net of issuance costs (in shares)				5,416,390
Issuance of common stock, net of issuance costs		$ 10,269		$ 1		$ 10,268
Issuance of common warrants with long-term debt, net	444				444
Issuance of common stock upon exercise of common warrants (in shares)			879,210
Issuance of common stock upon exercise of common warrants	3,485				3,485
Other financing fees	(133)				(133)
Stock-based compensation expense	1,296				1,296
Currency translation adjustment	(32)							(32)
Unrealized gains (losses) on marketable securities	0
Net income (loss)	(9,729)						(9,729)
Ending balance (in shares) at Sep. 30, 2020			107,235,976
Ending balance at Sep. 30, 2020	$ (7,382)		$ 11		386,374		(389,183)	(4,584)
Beginning balance (in shares) at Dec. 31, 2020	108,071,249		108,071,249
Beginning balance at Dec. 31, 2020	$ (18,006)		$ 11		391,175		(404,629)	(4,563)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)			34,696
Issuance of common stock under Employee Stock Purchase Plan	72				72
Issuance of common stock upon exercise of options (in shares)			153,278
Issuance of common stock upon exercise of options	244				244
Issuance of vested restricted stock units (in shares)			10,937
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)			4,706,844
Issuance of common stock through at-the-market offering, net	20,943				20,943
Stock-based compensation expense	1,780				1,780
Currency translation adjustment	(6)							(6)
Unrealized gains (losses) on marketable securities	(1)							(1)
Net income (loss)	(24,597)						(24,597)
Ending balance (in shares) at Mar. 31, 2021			112,977,004
Ending balance at Mar. 31, 2021	$ (19,571)		$ 11		414,214		(429,226)	(4,570)
Beginning balance (in shares) at Dec. 31, 2020	108,071,249		108,071,249
Beginning balance at Dec. 31, 2020	$ (18,006)		$ 11		391,175		(404,629)	(4,563)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	5
Unrealized gains (losses) on marketable securities	(1)
Net income (loss)	$ (37,926)
Ending balance (in shares) at Sep. 30, 2021	115,443,500		115,443,500
Ending balance at Sep. 30, 2021	$ (18,731)		$ 12		428,371		(442,555)	(4,559)
Beginning balance (in shares) at Mar. 31, 2021			112,977,004
Beginning balance at Mar. 31, 2021	(19,571)		$ 11		414,214		(429,226)	(4,570)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)			242,278
Issuance of common stock upon exercise of options	425				425
Issuance of vested restricted stock units (in shares)			10,938
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)			1,849,072
Issuance of common stock through at-the-market offering, net	8,563		$ 1		8,562
Stock-based compensation expense	1,783				1,783
Currency translation adjustment	12							12
Unrealized gains (losses) on marketable securities	1							1
Net income (loss)	4,565						4,565
Ending balance (in shares) at Jun. 30, 2021			115,079,292
Ending balance at Jun. 30, 2021	(4,222)		$ 12		424,984		(424,661)	(4,557)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)			24,098
Issuance of common stock under Employee Stock Purchase Plan	89				89
Issuance of common stock upon exercise of options (in shares)			1,936
Issuance of common stock upon exercise of options	5				5
Issuance of vested restricted stock units (in shares)			10,937
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)			327,237
Issuance of common stock through at-the-market offering, net	1,389				1,389
Stock-based compensation expense	1,904				1,904
Currency translation adjustment	(1)							(1)
Unrealized gains (losses) on marketable securities	(1)							(1)
Net income (loss)	$ (17,894)						(17,894)
Ending balance (in shares) at Sep. 30, 2021	115,443,500		115,443,500
Ending balance at Sep. 30, 2021	$ (18,731)		$ 12		$ 428,371		$ (442,555)	$ (4,559)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net loss	$ (37,926)	$ (53,430)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	851	490
Amortization of premiums and discounts on marketable securities	37	0
Non-cash lease expense	832	861
Loss on disposal of property and equipment	0	(39)
Stock-based compensation expense	5,467	4,186
Non-cash interest expense	800	406
Warrant liabilities revaluation	11,335	3,606
Loss on extinguishment of debt	0	461
Changes in operating assets and liabilities:
Accounts receivable	(100)	(7,626)
Prepaid expenses, deposits and other assets	(510)	(6,656)
Accounts payable	1,148	(29)
Income taxes payable	15,828	0
Deferred revenue	(28,424)	99,512
Accrued expenses and other liabilities	1,740	389
Net cash (used in) provided by operating activities	(28,922)	42,131
Cash flows from investing activities
Proceeds from maturities of marketable securities	6,400	0
Payment made for investments	(2,000)	0
Purchases of marketable securities	(30,455)	0
Purchases of property and equipment	(807)	(625)
Proceeds from the sale of property and equipment	0	50
Net cash used in investing activities	(26,862)	(575)
Cash flows from financing activities
Proceeds from issuance of long-term debt, net of expenses	0	24,838
Repayments of principal on outstanding debt	0	(19,313)
Net proceeds from issuance of common stock	30,906	2,137
Net proceeds from issuance of common stock- private placement	0	10,269
Issuance costs paid for December 2019 financing	0	(4,381)
Other financing fees	0	(192)
Proceeds from exercise of pre-funded and common warrants	0	978
Proceeds from exercise of stock options	674	101
Proceeds from issuance of common stock under Employee Stock Purchase Plan	161	184
Net cash provided by financing activities	31,741	14,621
Effect of exchange rate changes on cash	3	(88)
Net change in cash, cash equivalents, and restricted cash	(24,040)	56,089
Cash, cash equivalents, and restricted cash at beginning of period	140,064	91,551
Cash, cash equivalents, and restricted cash at end of period	116,024	147,640
Supplement cash flow information
Cash paid for interest	1,503	519
Noncash investing and financing activities
Cashless warrant exercise	0	18,228
Reclassification of warrant liability to equity upon exercise of warrants	0	1,494
Fair value of warrants issued in connection with issuance of long-term debt	0	444
Purchase of property and equipment not yet paid	17	17
Equity offering costs in accrued liabilities	11	117
Unrealized (losses) on marketable securities	(1)	0
Debt issuance costs in accrued liabilities	$ 0	$ 100

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc., or the Company, was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a clinical-stage biopharmaceutical company leveraging its ImmTOR™ immune tolerance platform with the goals of amplifying the efficacy of biologics, including enabling the re-dosing of life-saving gene therapies, and restoring self-tolerance in autoimmune diseases. The Company’s ImmTOR platform encapsulates rapamycin, also known as sirolimus, an immunomodulator, in biodegradable nanoparticles and is designed to induce antigen-specific immune tolerance. The Company believes ImmTOR has the potential to enhance the efficacy without compromising the safety of biologic therapies, improve product candidates under development, and enable novel therapeutic modalities. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three and nine months ended September 30, 2021 and 2020 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC, for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America, or U.S. GAAP, have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2020 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 12, 2021. The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Company’s financial position as of September 30, 2021, the consolidated results of operations for the three and nine months ended September 30, 2021, and cash flows for the nine months ended September 30, 2021. Such adjustments are of a normal and recurring nature. The results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021. Liquidity and Management’s Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain and sustain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital, which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through the initial public offering of its common stock, private placements of its common stock, issuances of common and preferred stock, debt, research grants and research collaborations. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, all of the Company’s revenue has been collaboration and grant revenue. The Company has devoted substantially all of its financial resources and efforts to developing its ImmTOR platform, identifying potential product candidates and conducting preclinical studies and clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any ImmTOR-enabled therapies. As of September 30, 2021, the Company’s cash, cash equivalents, restricted cash and marketable securities were $140.0 million, of which $1.4 million was restricted cash related to lease commitments and $0.3 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents, restricted cash and marketable securities as of September 30, 2021 will enable it to fund its operating expenses and capital expenditure requirements for at least twelve months from the issuance of these financial statements. As of September 30, 2021, the Company had an accumulated deficit of $442.6 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research and development of its product candidates and its administrative organization. The Company will require substantial additional financing to fund its operations and to continue to execute its strategy, and the Company intends to pursue a range of options to secure additional capital. At this time, any impact of COVID-19 on the Company’s business, revenues, results of operations and financial condition will largely depend on future developments, which are highly uncertain and cannot be predicted with confidence, such as the duration of the pandemic, travel restrictions and social distancing in the United States and other countries, business closures or business disruptions, supply chain disruptions, the ultimate impact on financial markets and the global economy, and the effectiveness of actions taken in the United States and other countries to contain and treat the disease. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through September 30, 2021, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The Company disclosed its significant accounting policies in Note 2 – Summary of Significant Accounting Policies included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020. There have been no material changes previously disclosed, with the exception of the matters discussed in recent accounting pronouncements. Recent Accounting Pronouncements Recently Adopted In October 2020, the FASB issued ASU 2020-10, Codification Improvements , which updates various codification topics by clarifying or improving disclosure requirements to align with the SEC’s regulations. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) – Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. Not Yet Adopted In May 2021, the FASB issued ASU 2021-04, Earnings Per Share (Topic 260), Debt – Modifications and Extinguishments (Subtopic 470-50), Compensation – Stock Compensation (Topic 718), and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40): Issuer's Accounting for Certain Modifications or Exchanges of Freestanding Equity-Classified Written Call Options . ASU 2021-04 provides guidance as to how entities should account for a modification of the terms or conditions or an exchange of a freestanding equity-classified written call option (i.e., a warrant) that remains equity-classified after modification or exchange as an exchange of the original instrument for a new instrument. An entity should apply the guidance provided in ASU 2021-04 prospectively to modifications or exchanges occurring on or after the effective date. This new standard will be effective for us for fiscal years beginning after December 15, 2021 including interim periods within those fiscal years. The adoption of ASU 2021-04 is not expected to have an impact on the Company’s financial position or results of operations upon adoption. In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40) . ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. This new standard will be effective for us for fiscal years beginning after December 15, 2021, including interim periods within those fiscal years. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments . Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses . ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, and reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This new standard will be effective for us for fiscal years beginning after December 15, 2021. The adoption of ASU 2016-13 is not expected to have an impact on the Company’s financial position or results of operations upon adoption.

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	Marketable Securities The following table summarizes the marketable securities held as of September 30, 2021 (in thousands): Amortized Unrealized gains Unrealized losses Fair September 30, 2021 Corporate bonds $ 2,035 $ — $ (1) $ 2,034 Commercial paper 21,984 — — 21,984 Total $ 24,019 $ — $ (1) $ 24,018 All marketable securities held at September 30, 2021 had maturities of less than 12 months when purchased and are classified as short-term marketable securities on the accompanying consolidated balance sheet. During the nine months ended September 30, 2021, there were no marketable securities adjusted for other than temporary declines in fair value. As of December 31, 2020, the Company held no marketable securities.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company has reported a net loss for the three and nine months ended September 30, 2021 and 2020. The Company used the treasury stock method to determine the number of dilutive shares. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per-share data): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Numerator: Net loss $ (17,894) $ (9,729) $ (37,926) $ (53,430) Denominator: Weighted-average common shares outstanding - basic and diluted 115,169,949 105,325,788 113,161,622 98,968,359 Net loss per share: Basic and diluted $ (0.16) $ (0.09) $ (0.34) $ (0.54) The following table represents the potential dilutive common shares excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Options, RSUs and ESPP shares 11,701,844 7,638,839 11,701,844 7,638,839 Warrants to purchase common stock 12,378,016 13,888,525 12,378,016 13,888,525 Total 24,079,860 21,527,364 24,079,860 21,527,364

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash equivalents) $ 56,563 $ 56,563 $ — $ — Marketable securities: Corporate bonds 2,034 — 2,034 — Commercial paper 21,984 — 21,984 — Total assets $ 80,581 $ 56,563 $ 24,018 $ — Liabilities: Warrant liabilities $ 40,043 $ — $ — $ 40,043 Total liabilities $ 40,043 $ — $ — $ 40,043 December 31, 2020 Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash equivalents) $ 80,576 $ 80,576 $ — $ — Total assets $ 80,576 $ 80,576 $ — $ — Liabilities: Warrant liabilities $ 28,708 $ — $ — $ 28,708 Total liabilities $ 28,708 $ — $ — $ 28,708 There were no transfers within the fair value hierarchy during the nine months ended September 30, 2021 or the year ended December 31, 2020. Cash, Cash Equivalents, and Restricted Cash As of September 30, 2021 and December 31, 2020, the money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase. As of September 30, 2021, the Company had restricted cash balances relating to a secured letter of credit in connection with its lease for the Company’s headquarters (see Note 8 included elsewhere in this Quarterly Report). The Company’s consolidated statement of cash flows includes the following as of September 30, 2021 and 2020 (in thousands): September 30, 2021 2020 Cash and cash equivalents $ 114,645 $ 146,261 Long-term restricted cash 1,379 1,379 Total cash, cash equivalents, and restricted cash $ 116,024 $ 147,640 Marketable Securities As of September 30, 2021, marketable securities classified as Level 2 within the valuation hierarchy consist of corporate bonds and commercial paper. Marketable securities represent holdings of available-for-sale marketable debt securities in accordance with the Company’s investment policy. The Company estimates the fair value of these marketable securities by taking into consideration valuations that include market pricing based on real-time trade data for the same or similar securities, and other observable inputs. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts. Loans Payable At September 30, 2021, in light of the recent issuance of the Term A Loan under the 2020 Term Loan, the Company believes the carrying value approximates the fair value of the loan. Common Warrants In December 2019, the Company issued common warrants in connection with a private placement of common shares. Pursuant to the terms of the common warrants, the Company could be required to settle the common warrants in cash in the event of certain acquisitions of the Company and, as a result, the common warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the common warrants upon issuance using the Black-Scholes valuation model and is required to revalue the common warrants at each reporting date with any changes in fair value recorded in the statement of operations and comprehensive loss. The valuation of the common warrants is considered Level 3 of the fair value hierarchy due to the need to use assumptions in the valuation that are both significant to the fair value measurement and unobservable including the volatility rate and the estimated term of the warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The changes in the fair values of the Level 3 warrant liability are reflected in the statement of operations and comprehensive loss for the three and nine months ended September 30, 2021 and 2020. The estimated fair value of warrants is determined using Level 3 inputs inherent in the Black-Scholes simulation valuation. Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the warrants is assumed to be equivalent to their remaining contractual term which expires on December 23, 2024. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility and the historical volatility of peer companies for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black-Scholes pricing model assumptions used to record the fair value of the warrant liability is as follows: September 30, 2021 Risk-free interest rate 0.53 % Dividend yield — Expected life (in years) 3.23 Expected volatility 98.07 % Changes in Level 3 Liabilities Measured at Fair Value on a Recurring Basis The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 10), for the nine months ended September 30, 2021 (in thousands): Warrant liabilities Fair value as of December 31, 2020 $ 28,708 Change in fair value 11,335 Fair value as of September 30, 2021 $ 40,043

Property and Equipment

Property and Equipment	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): September 30, December 31, 2021 2020 Laboratory equipment $ 5,095 $ 4,427 Computer equipment and software 734 532 Leasehold improvements 45 38 Furniture and fixtures 327 327 Office equipment 163 163 Construction in process 93 163 Total property and equipment 6,457 5,650 Less accumulated depreciation (4,650) (4,255) Property and equipment, net $ 1,807 $ 1,395 Depreciation expense was $0.1 million and $0.4 million for the three and nine months ended September 30, 2021, respectively. Depreciation expense was $0.1 million and $0.5 million for the three and nine months ended September 30, 2020, respectively.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): September 30, December 31, 2021 2020 Payroll and employee related expenses $ 2,552 $ 3,049 Collaboration and licensing 1,350 1,350 Accrued patent fees 457 534 Accrued external research and development costs 4,551 2,029 Accrued professional and consulting services 1,205 798 Accrued interest 165 170 Other 462 216 Accrued expenses $ 10,742 $ 8,146

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	Leases 65 Grove Street Lease In July 2019, the Company entered into a lease for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Headquarters Lease. As part of the Headquarters Lease, the Company incurred $0.8 million in non-reimbursable construction costs. The lease began in March 2020, when the Company took control of the office space, and the lease term is 8 years. The discount rate of 8.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods. In connection with the Headquarters Lease, the Company secured a letter of credit from Silicon Valley Bank, or SVB, for $1.4 million, recognized as long-term restricted cash, as of September 30, 2021 and December 31, 2020, respectively, which automatically renews each year. Moscow, Russia Lease The Company has a month-to-month facility agreement for its Moscow, Russia office. Rent expense is recognized as incurred. For the three and nine months ended September 30, 2021 and 2020 the components of lease costs were as follows (in thousands): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Operating lease cost $ 457 $ 506 $ 1,355 $ 1,590 Variable lease cost 182 123 652 496 Short-term lease cost 2 3 7 8 Total lease cost $ 641 $ 632 $ 2,014 $ 2,094 The maturity of the Company’s operating lease liabilities as of September 30, 2021 were as follows (in thousands): September 30, 2021 2021 (remainder) $ 457 2022 1,866 2023 1,922 2024 1,980 2025 2,039 Thereafter 4,945 Total future minimum lease payments 13,209 Less imputed interest 3,323 Total operating lease liabilities $ 9,886 The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): September 30, 2021 2020 Cash paid for amounts included in the measurement of lease liabilities: $ 1,355 $ 2,079 Other than the initial recording of the right-of-use asset and lease liability for the Headquarters Lease in 2020, which was non-cash, the changes in the Company’s right-of-use asset and lease liability for the nine months ended September 30, 2021 and 2020 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: September 30, 2021 2020 Weighted-average remaining lease term 6.6 years 7.7 years Weighted-average discount rate 8.9 % 8.9 %

Debt

Debt	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt	Debt 2020 Term Loan On August 31, 2020, the Company entered into a term loan of up to $35.0 million, or the 2020 Term Loan, consisting of term loans in an aggregate amount of $25.0 million, or the Term A Loan, and term loans in an aggregate amount of $10.0 million, or the Term B Loan, governed by a loan and security agreement, or the Loan Agreement, between the Company and Oxford Finance LLC, or Oxford, as Collateral Agent and a Lender, and SVB, as a Lender. The Term A Loan was funded in full on August 31, 2020, or the Funding Date. The Term B Loan was to be available, subject to Collateral Agent’s discretion and customary terms and conditions, during the period commencing on the date the Company had delivered to the Collateral Agent and the Lenders evidence: (i) the Company or one of the Company’s collaboration partners has enrolled its first patient for a Phase 1 clinical trial evaluating the treatment of methylmalonic acidemia, or MMA, and (ii) the Company has enrolled the first patient in each of two Phase 3 pivotal trials evaluating SEL-212, or the Second Draw Period Milestone, and ending on the earliest of (i) the date which is 30 days following the date the Second Draw Period Milestone is achieved, (ii) September 30, 2021 and (iii) the occurrence of an event of default, other than an event of default that has been waived in writing by Collateral Agent and the Lenders in their sole discretion, with such period referred to as the Second Draw Period. The Second Draw Period expired on September 30, 2021 and the Term B Loan is no longer available to be drawn by the Company in the future. The 2020 Term Loan will mature on August 1, 2025. Each advance under the Term Loan accrues interest at a floating per annum rate equal to the greater of (a) 7.90%, and (b) the lesser of (x) the sum of (i) the prime rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, and (ii) 4.65% and (y) 10.00%. The Term Loan provides for interest-only payments on a monthly basis until April 1, 2022. Thereafter, amortization payments will be payable monthly in equal installments of principal and interest to fully amortize the outstanding principal over the remaining term of the loan, subject to recalculation upon a change in the prime rate. The Company may prepay the Term Loan in full but not in part provided that the Company (i) provides ten days’ prior written notice to Collateral Agent, (ii) pays on the date of such prepayment (A) all outstanding principal plus accrued and unpaid interest, and (B) a prepayment fee of between 3.0% and 1.0% of the aggregate original principal amount advanced by the lender depending on the timing of the prepayment. Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 5.0% per annum of the past due amount outstanding. At the end of the loan term (whether at maturity, by prepayment in full or otherwise), the Company shall make a final payment to the lender in the amount of 9.0% of the aggregate original principal amount advanced by the lender. The final payment fee totaling $2.3 million is recorded as a loan discount. The Term Loan is secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets includes any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company has also granted the Collateral Agent a negative pledge with respect to its intellectual property. The Loan Agreement contains customary covenants and representations, including but not limited to financial reporting obligations and limitations on dividends, indebtedness, collateral, investments, distributions, transfers, mergers or acquisitions, taxes, corporate changes, deposit accounts, and subsidiaries. The Loan Agreement also contains other customary provisions, such as expense reimbursement, non-disclosure obligations as well as indemnification rights for the benefit of the Collateral Agent. The events of default under the Loan Agreement include, but are not limited to, the Company’s failure to make any payments of principal or interest under the Loan Agreement or other transaction documents, the Company’s breach or default in the performance of any covenant under the Loan Agreement or other transaction documents, the occurrence of a material adverse change, the Company making a false or misleading representation or warranty in any material respect under the Loan Agreement, the Company’s insolvency or bankruptcy, any attachment or judgment on the Company’s assets of at least $0.5 million, or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of $0.5 million. If an event of default occurs, the Collateral Agent is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement. The Company incurred $0.4 million in debt issuance costs in connection with the closing of the 2020 Term Loan. Debt issuance costs are presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt. The Company assessed all terms and features of the 2020 Term Loan to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the 2020 Term Loan, including any put, call, and contingent features. The Company determined that the interest rate collar and prepayment call option did not require bifurcation; whereas the contingent put option and default (contingent) interest rate feature met bifurcation criteria resulting in immaterial amounts. Warrants On August 31, 2020, in connection with the 2020 Term A Loan, the Company issued warrants to the Lenders to purchase an aggregate of 196,850 shares of its common stock at an exercise price equal to $2.54 per share. In accordance with ASC 815-40, these warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance. The initial grant date fair value of the warrants was $0.4 million as determined by the Black-Scholes valuation model and recorded to stockholders' equity, with the SVB portion allocated to the reacquisition price of the 2017 Term Loan and the Oxford fair value portion as a loan discount to the Term A Loan. Additionally, on August 31, 2020, pursuant to the terms of a Warrant Side Letter agreement among the Company and the Lenders, the Company agreed to issue to the Lenders, on the date the Company draws the Term B Loan and in accordance with each party’s respective pro rata share with respect to the Term B Loan, one or more warrants to purchase an aggregate number of shares of its common stock that is equal to $200,000 divided by the average closing price of the Company’s common stock on The Nasdaq Stock Market LLC for the ten consecutive trading days ending the day before such issuance, rounded down to the nearest whole number of shares, and having an exercise price equal to the Term B Warrant Price. As the Company is no longer able to draw the Term B Loan due to the expiration of the Second Draw Period, this additional warrant issuance is no longer possible. Payoff On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which the Company utilized $13.7 million of the 2020 Term Loan to pay off all outstanding obligations under the previous term loan, consisting of the principal payment, final prepayment and accrued interest. During the three and nine months ended September 30, 2020, the Company recognized a loss on extinguishment of debt in the amount of $0.5 million determined as the difference between the reacquisition price and carrying value at August 31, 2020. As of September 30, 2021 and December 31, 2020, the outstanding principal balance under the 2020 Term Loan was $25.0 million. Future minimum principal and interest payments on the 2020 Term Loan as of September 30, 2021 are as follows (in thousands): 2021 (remainder) $ 499 2022 7,343 2023 8,611 2024 8,027 2025 7,274 Total minimum debt payments 31,754 Less: Amount representing interest (4,505) Less: Debt discount and deferred charges (1,820) Less: Current portion of loan payable (4,125) Loan payable, net of current portion $ 21,304

Equity

Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Equity	Equity Equity Financings August 2020 Shelf Registration Statement On August 6, 2020, the Company filed an updated universal shelf registration statement on Form S-3 (Reg. No. 333-241692) with the SEC to sell an aggregate amount of up to $200.0 million of certain of its securities. The shelf registration statement was declared effective by the SEC on August 14, 2020. “At-the-Market” Offerings 2017 Sales Agreement In August 2017, the Company entered into a sales agreement, or the 2017 Sales Agreement, with Jefferies LLC, as sales agent, to sell shares of its common stock with an aggregate value of up to $50.0 million in an “at the market offering.” On August 6, 2020, concurrent with the filing of the updated shelf registration statement, the Company entered into a sales agreement, or the 2020 Sales Agreement with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50.0 million in an “at the market offering.” The 2017 Sales Agreement terminated pursuant to its terms in August 2020. During the nine months ended September 30, 2021, the Company sold 6,883,153 shares of its common stock pursuant to the 2020 Sales Agreement for aggregate net proceeds of $30.9 million, after deducting commissions and other transaction costs. During the year ended December 31, 2020, the Company sold 1,069,486 shares of its common stock pursuant to the 2020 and 2017 Sales Agreements at an average price of approximately $2.16 per share for aggregate net proceeds of $2.1 million, after deducting commissions and other transaction costs. On October 8, 2021, the Company delivered notice to Jefferies LLC that the Company was terminating the 2017 Sales Agreement, with effect as of October 19, 2021. Refer to Note 18 Subsequent Events. June 2020 Sobi Stock Purchase On June 11, 2020, the Company entered into a stock purchase agreement with Sobi, pursuant to which the Company sold an aggregate of 5,416,390 shares of its common stock at a purchase price equal to $4.6156 per share, which represented 120% of the 10-day volume-weighted average price of the Company’s common stock prior to signing, for aggregate gross proceeds of $25.0 million, or the Sobi Private Placement. The closing of the Sobi Private Placement occurred on July 31, 2020. The shares of common stock acquired in the Sobi Private Placement are subject to a one-year lock-up from closing, during which time Sobi is prohibited from selling or otherwise disposing of such shares. In accordance with ASC 815, this forward sale treatment qualified as equity classification as the shares are not within the scope of ASC 480. The gross proceeds of $25.0 million were determined to include a premium to the fair value of the Company’s shares as of July 28, 2020 of approximately $14.5 million. As a result, such amount was included in the transaction price for revenue recognition of the Sobi License. See Note 12 for details. Also on June 11, 2020, the Company entered into a registration rights agreement (as amended by that certain letter agreement, dated as of November 4, 2020) with Sobi, pursuant to which the Company agreed to prepare and file a registration statement with respect to the resale of the shares of common stock acquired in the Sobi Private Placement. The Company will be required to file this resale registration statement within 30 days following receipt by the Company of a written request from Sobi to file such resale registration statement, and to have the registration statement declared effective within 10 business days after the SEC informs the Company that no review of such resale registration statement will be made or that the SEC has no further comments on such resale registration statement. December 2019 Financing On December 18, 2019, the Company entered into a securities purchase agreement, or the 2019 Purchase Agreement, with a group of institutional investors and certain members of the Board of Directors. Pursuant to the 2019 Purchase Agreement, the Company sold an aggregate of 37,634,883 shares of its common stock at a purchase price of $1.46 per share, warrants to purchase an aggregate of 22,988,501 shares of common stock at a purchase price of $0.125 per share underlying each common warrant, and pre-funded warrants to purchase an aggregate of 8,342,128 shares of common stock at a purchase price of $1.46 per share, all with five year terms, or the 2019 PIPE. The closing of the 2019 PIPE occurred on December 23, 2019. The exercise price of the pre-funded warrants is $0.0001 per share and the exercise price for the common warrants is $1.46 per share. In the event of a certain sale of the Company, the terms of the common warrants require us to make a payment to such common warrant holders based on a Black-Scholes valuation (using variables as specified in the warrants). This provision does not apply to the pre-funded warrants. Therefore, the Company is required to account for the common warrants as liabilities and record them at fair value, while the pre-funded warrants met the criteria to be classified as permanent equity. The Company recorded the fair value of the common warrants of $40.7 million upon issuance using the Black-Scholes valuation model. Issuance costs were allocated between the equity component with an offset to additional paid-in capital and the liability component recorded as expense on a relative fair value basis. Total net proceeds from the equity offering was $65.6 million, after deducting transaction costs and commissions of $4.4 million which was paid in the three months ended March 31, 2020. The common warrants were revalued as of September 30, 2021 at $40.0 million. During the three months ended September 30, 2021 and 2020, the Company recorded a decrease in the fair value of the warrants of $0.6 million and $4.8 million, respectively, in the unaudited consolidated statements of operations and comprehensive loss. During the nine months ended September 30, 2021 and 2020, the Company recorded an increase in the fair value of the warrants of $11.3 million and $3.6 million, respectively, in the unaudited consolidated statements of operations and comprehensive loss. Warrants During the nine months ended September 30, 2021, there were no warrants issued, exercised, or canceled. Number of Warrants Equity Liability classified Total Weighted average Outstanding at September 30, 2021 292,469 12,085,547 12,378,016 $ 1.60 Reserved Shares The Company has authorized shares of common stock for future issuance as follows: As of September 30, 2021 December 31, 2020 Exercise of common warrants 12,378,016 12,378,016 Shares available for future stock incentive awards 6,049,422 4,916,374 Unvested restricted stock units 561,888 87,500 Outstanding common stock options 11,082,277 7,775,249 Total 30,071,603 25,157,139

Stock Incentive Plans

Stock Incentive Plans	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock Incentive Plans	Stock Incentive Plans The Company maintains the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board. In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 1,210,256 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expire, lapse or terminate become available under the 2016 Plan as shares available for future grants. Additionally, pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2021 and 2020, the number of shares of common stock that may be issued under the 2016 Plan was increased by 4,322,850 and 3,453,022 shares, respectively. As of September 30, 2021, 1,935,395 shares remain available for future issuance under the 2016 Plan. In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan was adopted by the Board without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 1,175,000 shares of its common stock for issuance. In March 2019, the Board approved the amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of September 30, 2021, there are 1,591,661 shares available for future grant under the 2018 Inducement Incentive Award Plan. Stock-based Compensation Expense Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Research and development $ 791 $ 532 $ 2,331 $ 1,773 General and administrative 1,113 764 3,136 2,413 Total stock-based compensation expense $ 1,904 $ 1,296 $ 5,467 $ 4,186 Stock Options Employees The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Risk-free interest rate 1.01 % 0.24 % 0.79 % 1.19 % Dividend yield — — — — Expected term 6.06 6.08 6.03 6.05 Expected volatility 93.55 % 94.88 % 95.05 % 90.19 % Weighted-average fair value of common stock $ 4.19 $ 2.47 $ 3.58 $ 2.39 The weighted average grant date fair value of stock options granted to employees during the three and nine months ended September 30, 2021 and 2020 was $3.17 and $1.88, $2.73 and $1.77 respectively. As of September 30, 2021, total unrecognized compensation expense related to unvested employee stock options was $12.8 million, which is expected to be recognized over a weighted average period of 2.9 years. Non-employee consultants As of September 30, 2021, there was no unrecognized compensation expense related to non-employee consultants’ stock options. The following table summarizes the stock option activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan: Weighted-average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employees Outstanding at December 31, 2020 7,302,176 $ 3.98 8.43 $ 4,456 Granted 4,609,811 $ 3.58 Exercised (397,492) $ 1.72 Forfeited (905,291) $ 2.82 Outstanding at September 30, 2021 10,609,204 $ 3.99 8.44 $ 11,080 Vested at September 30, 2021 3,583,316 $ 5.27 7.32 $ 4,133 Vested and expected to vest at September 30, 2021 9,775,480 $ 4.04 8.36 $ 10,400 Non-employee consultants Outstanding at December 31, 2020 473,073 $ 5.89 5.23 $ 86 Granted — $ — Exercised — $ — Forfeited — $ — Outstanding at September 30, 2021 473,073 $ 5.89 4.48 $ 282 Vested at September 30, 2021 473,073 $ 5.89 4.48 $ 282 Vested and expected to vest at September 30, 2021 473,073 $ 5.89 4.48 $ 282 Restricted Stock Units In January 2021, the Company granted 369,800 restricted stock awards to employees under the 2016 Plan, which will vest over a four year term. In addition, during the first quarter of 2021, the Company awarded 197,500 restricted stock units to executives under the 2016 Plan, of which 98,750 were determined to be granted as of the award date consistent with ASC 718. On September 24, 2021, the Compensation Committee of the Board of Directors, under authority duly granted to them by the Board of Directors, determined a definitive performance metric for the second performance condition previously undefined, and therefore granted the previously reserved 98,750 restricted stock units. These restricted stock units will vest in two equal installments on the dates an applicable performance condition is achieved, on or prior to December 31, 2021. If the performance conditions are not satisfied on or prior to December 31, 2021, the restricted stock units will be forfeited for no consideration. The restricted stock units granted during the first quarter of 2021 had a weighted average fair value of $2.99 per share based on the closing price of the Company’s common stock on the date of grant. The restricted stock units were valued at approximately $1.4 million on their grant date. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock awards to employees based on historical attrition trends. In September 2021, the Company granted 36,300 restricted stock awards to executives under the 2016 Plan which will vest over a four-year term. Unrecognized compensation expense for all restricted stock units was $1.4 million as of September 30, 2021, which is expected to be recognized over a weighted average period of 2.1 years. The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan: Number of shares Weighted average Unvested at December 31, 2020 87,500 $ 6.03 Granted 603,600 3.24 Vested (32,812) 6.03 Forfeited (96,400) 2.99 Unvested at September 30, 2021 561,888 $ 3.56 Employee Stock Purchase Plan In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 173,076 shares of common stock for future issuance under the ESPP to participating employees. In January 2021 and 2020, the number of shares of common stock authorized for issuance under the ESPP was increased by 1,080,711 shares and 863,254 shares, respectively. During the nine months ended September 30, 2021, the Company issued 58,794 shares of common stock under the ESPP. As of September 30, 2021, 2,522,366 shares remain available for future issuance under the ESPP. For each of the three and nine months ended September 30, 2021 and 2020, the Company recognized less than $0.1 million and $0.1 million of stock-based compensation expense under the ESPP, respectively.

Revenue Arrangements

Revenue Arrangements	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements Swedish Orphan Biovitrum License and Development Agreement On June 11, 2020, the Company and Sobi entered into the Sobi License. Pursuant to the Sobi License, the Company has agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the Company’s SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, up-front payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. Pursuant to the Sobi License, the Company has agreed to supply (at cost) quantities of the Compound and ImmTOR as necessary for completion of the two Phase 3 clinical trials of SEL-212 (DISSOLVE I and DISSOLVE II) and a 6-month placebo extension. The Company is required to supply quantities of the Compound until all rights to the Compound and any materials needed to manufacture the Compound are transferred to Sobi. Sobi has agreed to reimburse the Company for all budgeted costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, except for any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212. Sobi will have control and responsibility over all regulatory filings, including any investigational drug applications (IND), biologics license applications (BLA), and marketing authorization applications (MAA) relating to the licensed product. The transactions contemplated by the Sobi License were consummated on July 28, 2020 following the expiration or termination of the required waiting periods under the Hart-Scott-Rodino Antitrust Improvements Act of 1976. Sobi may terminate the Sobi License for any reason upon 180 days’ written notice to the Company, whereby all rights granted under the Sobi License would revert back to the Company. In addition, if Sobi were to terminate the Sobi License, the Company has the option to obtain a license to all patents and know-how necessary to exploit SEL-212 in existence as of the termination date from Sobi in return for making an equitable royalty payment to Sobi. Additionally, on June 11, 2020, the Company entered into the Sobi Purchase Agreement in connection with the Sobi License. The closing of the Sobi Private Placement occurred on July 31, 2020, following the closing of the transactions contemplated under the Sobi License. See Note 10 for details. The Company determined that the Sobi License represents a service arrangement under the scope of ASC 606. In addition, given the Sobi License and Sobi Purchase Agreement were executed contemporaneously and negotiated as a package with a single commercial objective, the Company will account for the two agreements as a single contract. The term of the Sobi License commenced upon the effective date of July 28, 2020 and will continue on a product-by-product basis until the royalty terms for each country have expired. The royalty term for a given product begins upon the first commercial sale of the product in a country and ends at the later of ten years from the first commercial sale, expiration of the last valid patent claim covering the product and expiration of all regulatory exclusivity periods for the product in a country. Given the reversion of the rights under the Sobi License represents a penalty in substance for a termination by Sobi, the contract term would remain the stated term of the Sobi License. The Company determined that the Sobi License contains three distinct performance obligations due to the nature of the promises in the contract, which includes conducting the Phase 3 DISSOLVE trials, Sobi’s option to set-up a second source supplier, and a combined obligation comprised of the delivery of the license to SEL-212, transfer of the know-how and the manufacturing and delivery of SEL-212 supply for development, or the Combined License Obligation. As the set-up of a second source supplier is optional for Sobi and the Company will be reimbursed at cost for its efforts in the subsequent set-up and technology transfer, the option for this future service was determined to be at a significant and incremental discount to its standalone selling price and treated as a material right in the arrangement, namely a distinct performance obligation. In determining the transaction price, the Company concluded the upfront payment of $75.0 million and the $5.0 million development milestone associated with the dosing of the first patient in the Phase 3 DISSOLVE trials will be included in the transaction price. All other development milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of the evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company and probability of success criteria is estimated. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. In connection with the Sobi Purchase Agreement, the Company determined that the gross proceeds of $25.0 million from the Sobi Private Placement included a premium to the fair value of the Company’s shares as of July 28, 2020 equal to approximately $14.5 million. The premium amount will be included in the transaction price for revenue recognition. The Company will estimate and include in the transaction price the total reimbursements to be received from Sobi for both the manufacturing and delivery of the Compound and ImmTOR as well as conducting the Phase 3 DISSOLVE trials. The Company determined that a significant financing component does not exist in its arrangement with Sobi. The Company allocated the transaction price based on the relative standalone selling prices of the three distinct performance obligations. The Company estimated the standalone selling price of conducting the Phase 3 DISSOLVE trials by forecasting its anticipated costs and applying a margin reflective of the industry. The Company must determine the standalone selling price of the second source supplier option by determining the discount given to Sobi multiplied by the likelihood that Sobi will exercise the option in the future. Similar to the Phase 3 program estimate, the Company estimated the discount of the option by forecasting the set-up costs and applying a margin that is reflective of the industry. As the Company will be providing the set-up and technology transfer services and the future supply at cost, the discount of the option is equal to the margin amount. The Company considered discussions with Sobi as well as probability of regulatory success of SEL-212 in determining the likelihood of exercise. The Company estimated the standalone selling price of the Combined License Obligation by utilizing a discounted cash flow model. The Company determined that the delivery of the supply to Sobi best represents the pattern of delivery of the Combined License Obligation as the supply is essential to the utility of the license and know-how. The Company will recognize the revenue allocated to the Combined License Obligation by utilizing the output method. The Company estimated the total supply of the Compound and ImmTOR to be required during the clinical trial period and will recognize revenue as this supply is shipped for use in the clinical trials. The Company will recognize the revenue allocated to the conducting of the Phase 3 DISSOLVE trials obligation by utilizing the input method. The Company estimated the total budgeted costs to be incurred over the Phase 3 DISSOLVE trials and will recognize revenue as these costs are incurred. The Company’s costs best represent the pattern of transfer as these will capture all performance of the trials completed to date and can be readily measured. The Company will recognize the revenue allocated to the second source supplier option when the future services and goods are transferred. As of September 30, 2021 and December 31, 2020, the Company recorded $55.9 million and $68.3 million, respectively, as a short-term contract liability and $5.5 million and $24.2 million, respectively, as a long-term contract liability, representing deferred revenue associated with this agreement. In addition, as of September 30, 2021 the Company has recorded $1.0 million of contract assets related to incremental costs that would not have been incurred if the Sobi License had not been obtained, of which $0.9 million is presented in prepaid expenses and other current assets and $0.1 million is presented in other assets in the accompanying unaudited consolidated balance sheets. Amortization of contract assets was $0.4 million for the nine months ended September 30, 2021. As of September 30, 2021 and December 31, 2020, the Company recorded a total outstanding receivable of $7.0 million and $6.9 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Revenue of $24.3 million and $54.8 million related to the Sobi License was recognized during the three and nine months ended September 30, 2021, respectively. Revenue of $4.3 million related to the Sobi License was recognized during the three and nine months ended September 30, 2020. Sarepta Therapeutics, Inc. Research License and Option Agreement On June 13, 2020, the Company and Sarepta entered into a Research License and Option Agreement, or the Sarepta Agreement. Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes, or the Indications. Sarepta will have an option term of 24 months during which it can opt-in to obtain an exclusive license to further develop and commercialize the Product to treat at least one Indication, with a potential to extend the option term for an additional fee. The Company will supply ImmTOR to Sarepta for clinical supply on a cost-plus basis. Sarepta paid a $2.0 million up-front payment to the Company upon signing of the Sarepta Agreement, and the Company is eligible to receive additional preclinical payments during the option term. If Sarepta opts-in to an exclusive license agreement, the Company could receive option exercise payments per Indication upon execution of the exclusive license, and the Company would be entitled to significant development and commercial milestone payments and tiered royalties ranging from the mid-to-high single digits based on net sales. Pursuant to the Sarepta Agreement, the Company determined the Sarepta Agreement represents a service arrangement under the scope of ASC 606, with a 24 month contract duration. Given the reversion of the rights under the Sarepta Agreement represents a penalty in substance for a termination by Sarepta, the contract term would remain the stated term of the Sarepta Agreement. The Company determined that the Sarepta Agreement and supply obligation including the delivery of the research license, the licensed know-how, the manufactured supply and delivery of materials represent a single promise and performance obligation to be transferred to Sarepta over time due to the nature of the promises in the contract. The delivery of the manufactured supply is the predominant promise within the arrangement, as it is essential to the utility of the licensed intellectual property. As such, consideration in the initial transaction price will be allocated to the single performance obligation based on the contractual price. In determining the transaction price, the Company concluded the payment associated with all the performance milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such study milestones is outside the control of the Company and probability of success criteria is estimated. The Company also determined the option to enter into a future commercial license agreement and extend the term of the option does not represent a material right since it was not priced at an incremental discount. Sarepta may terminate the Sarepta Agreement for any reason upon 30 days’ written notice to the Company. The Sarepta Agreement contains other customary terms and conditions, including representations and warranties, covenants, termination, and indemnification obligations in favor of each party. During the year ended December 31, 2020, the Company and Sarepta entered into two amendments relating to an additional feasibility study. During the nine months ended September 30, 2021, the Company and Sarepta entered into a third amendment relating to the additional feasibility study. On April 13, 2021, the Company was notified by Sarepta of the achievement of the milestone event related to the completion of a non-clinical study for Duchenne muscular dystrophy and certain limb-girdle muscular dystrophies under the Sarepta Agreement. Accordingly, the Company received a milestone payment of $3.0 million during the three months ended June 30, 2021. As of September 30, 2021, two milestones remained constrained, and as of December 31, 2020, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved. The Company will recognize the revenue associated with the upfront payment and combined single performance obligation utilizing the output method, over the 24 month term as the manufactured supply is delivered to Sarepta. As of September 30, 2021 and December 31, 2020, the Company recorded $4.8 million and $2.0 million, respectively, as a short-term contract liability representing deferred revenue associated with this agreement. Revenue of $0.2 million related to the Sarepta Agreement was recognized during each of the three and nine months ended September 30, 2021. Revenue of $0.3 million related to the Sarepta License Agreement was recognized during the three and nine months ended September 30, 2020. Asklepios Biopharmaceutical, Inc. License Agreement for Pompe Disease On December 17, 2019, the Company and AskBio entered into a license agreement, or the AskBio License Agreement. Pursuant to the AskBio License Agreement, AskBio has exercised its option to exclusively license the Company’s intellectual property rights covering the Company’s ImmTOR platform to research, develop, and commercialize certain AAV gene therapy products utilizing ImmTOR, and targeting the GAA gene, or derivatives thereof, to treat Pompe Disease. Pursuant to the AskBio License Agreement and ancillary documents, AskBio agreed to pay to the Company upfront fees of an aggregate of $7.0 million. Assuming successful development and commercialization, the Company could receive up to an additional $237.0 million in development, regulatory, and sales milestone payments. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-to-high single digits. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product until the expiration of the later of (i) ten years after the first commercial sale and (ii) expiration of the last to expire valid claim on patents covering the licensed product. Pursuant to the AskBio License Agreement, the Company will supply AskBio with its ImmTOR platform, or the Supply Obligation, and AskBio will be responsible for all preclinical, clinical and commercial manufacture and supply of licensed products (other than ImmTOR) and carry out all other activities related to the research, development, and commercialization of licensed products at its sole expense, including all regulatory activities related thereto. The Company determined that the AskBio License Agreement and Supply Obligation represent a single promise and performance obligation. This is because AskBio cannot derive benefit from the license without the simultaneous transfer of the patent protected ImmTOR supply. Therefore, the License Obligation and Supply Obligation represent the only promise in the arrangement and are combined as a single performance obligation. In determining the transaction price, the Company concluded that the future development milestones, regulatory milestones, sales milestones, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Consideration related to sales-based milestones as well as royalties on net sales upon commercialization by AskBio, will be recognized when the related sales occur, as they were determined to relate predominantly to the intellectual property granted to AskBio and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of September 30, 2021 and December 31, 2020, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. The total initial transaction price of the contract on the effective date was $7.0 million, comprised of a $2.0 million initial up-front payment upon agreement of terms, and a $5.0 million initial up-front execution fee. At each of September 30, 2021 and December 31, 2020, the Company recorded $1.7 million as short-term contract liability and $5.3 million as a long-term contract liability, representing deferred revenue associated with this agreement. Revenue will be recognized over the period in which the particles are delivered. No revenue related to the AskBio License Agreement was recognized during the three and nine months ended September 30, 2021 and 2020 as no deliveries were made during these periods. Spark Therapeutics, Inc. Spark License Agreement The disclosures relating to the Company’s license and option agreement, or the Spark License Agreement, with Spark pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR platform reported in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 which was filed with the SEC on March 12, 2021 have not materially changed since the Company filed such report. Skolkovo Foundation During the nine months ended September 30, 2021, revenue of $0.1 million related to the remaining contract liability of the Russia-based Development Fund of New Technologies Development and Commercialization Center, or Skolkovo, grant funding was recognized at the expiration of the three-year audit period. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed). As of September 30, 2021, the aggregate amount of the transaction price allocated to remaining performance obligations was $82.4 million. Contract Balances from Contracts with Customers ( Sobi, Sarepta, AskBio, Spark and Skolkovo ) The following table presents changes in the Company’s contract liabilities during the nine months ended September 30, 2021 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Nine Months Ended September 30, 2021 Contract liabilities: Deferred revenue $ 110,796 $ 3,000 $ (31,424) $ 82,372 Total contract liabilities $ 110,796 $ 3,000 $ (31,424) $ 82,372

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related-Party Transactions	Related-Party Transactions Consulting Services The Company incurred expenses for consulting services provided by its founders totaling less than $0.1 million and $0.1 million during the three and nine months ended September 30, 2021 and 2020, respectively. The Company entered into consulting agreements with its founders to serve on its Scientific Advisory Board, effective January 1, 2020 to December 31, 2021, under which they will be paid quarterly for their services.

Collaboration and License Agree

Collaboration and License Agreements	9 Months Ended
	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration and License Agreements	Collaboration and License Agreements Cyrus Biotechnology, Inc. Collaboration and License Agreement On September 7, 2021, the Company and Cyrus Biotechnology, Inc., or Cyrus, entered into a collaboration and license agreement, or the Cyrus Agreement. Pursuant to the Cyrus Agreement, Cyrus agreed to grant the Company an exclusive, worldwide license to certain intellectual property to form a protein engineering collaboration combining the Company’s ImmTOR platform with Cyrus’ ability to redesign protein therapeutics. The lead program is a proprietary interleukin-2, or IL-2, protein agonist designed to selectively promote expansion of regulatory T cells for treatment of patients with autoimmune diseases and other deleterious immune conditions. Cyrus will contribute its expertise to apply its Rosetta-based computational platform to design candidate variants. Cyrus will also experimentally screen and characterize candidate variants for expression, stability, and receptor binding. The Company will experimentally characterize lead candidates for regulatory T cell expansion and activation in vitro and in vivo. In return for the licensed intellectual property, the Company is obligated to make an upfront payment and pay certain discovery, development, and sales-based milestones which could potentially total up to approximately $1.5 billion across multiple programs. The Cyrus Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk is not shared by both parties. The Company will expense costs related to the Cyrus Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Cyrus tiered royalties ranging from mid-single digit to low-double digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. Additionally, on September 7, 2021, the Company entered into a stock purchase agreement, or the Series B Preferred Stock Purchase Agreement, in connection with the Cyrus Agreement. Pursuant to the Series B Preferred Stock Purchase Agreement, the Company purchased 2,326,934 shares of Cyrus’ Series B Preferred Stock, par value $0.0001 per share at a purchase price of $0.8595 per share for $2.0 million. In accordance with ASC 810, the Company has a variable interest in Cyrus resulting from its equity investment. The Company will share in Cyrus’ expected losses or receive a portion of its expected returns and absorb the variability associated with changes in the entity’s net assets. However, the Company is not the primary beneficiary as it does not have the power to direct the activities most significant to Cyrus, and therefore it is not required to consolidate Cyrus. The Company determined its equity interest to be within the scope of ASC 321 and elected to record the $2.0 million investment of Cyrus’ Series B Preferred Stock at cost on the purchase date. As of September 30, 2021, no impairment indicators are present and therefore the carrying value of the investment in Cyrus is $2.0 million on the accompanying consolidated balance sheet. The Company’s maximum exposure to loss related to this variable interest entity is limited to the carrying value of the investment. The Company has not provided financing to Cyrus other than the amount contractually required by the Series B Preferred Stock Purchase Agreement. Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement In August 2019, the Company entered into a feasibility study and license agreement with AskBio, or the AskBio Collaboration Agreement. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio’s AAV gene therapy, or SEL-302, (previously disclosed as MMA-101, in combination with ImmTOR) for the treatment of MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies, or the POC Studies. On April 29, 2021, the Company was notified by AskBio that it intended to opt-out of development of the MMA indication. Consequently, the Company will assume all rights to the MMA program and intends to continue to progress the SEL-302 program through clinical development. The SEL-399 program combines an empty AAV capsid (EMC-101), which is an AAV capsid containing no transgene, with ImmTOR and is being conducted in partnership with AskBio. Building on the preclinical data the Company has generated showing ImmTOR’s effect on mitigating or reducing the formation of neutralizing antibodies to AAV gene therapies, the Company has commenced a clinical trial of SEL-399 in healthy adult volunteers in Belgium. The goal of the SEL-399 clinical trial is to demonstrate the appropriate dose of ImmTOR in humans to mitigate the formation of antibodies to AAV capsids used in gene therapies. The Company and AskBio will share responsibility for the research, development and commercialization of products developed under the SEL-399 program collaboration. The parties will also share research, development, and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby no longer be required to share costs for such products. Each party will receive a percentage of net profits under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio is responsible for manufacturing the AAV capsids and AAV vectors and the Company is responsible for manufacturing ImmTOR. The AskBio Collaboration Agreement is considered to be within the scope of ASC 808, as both parties are active participants and exposed to the risks and rewards of the collaborative activity. The Company evaluated the terms of the AskBio Collaboration Agreement and have identified the following promises in the arrangement (1) conducting research and development activities to develop and commercialize products under the collaboration, or the R&D Services, (2) granting a non-exclusive, non-transferable, royalty-free, fully paid up, worldwide license to certain intellectual property of the Company, or the IP Rights, for the purpose of performing the POC Studies, or the Research License, (3) granting an exclusive, nontransferable, worldwide license to the IP Rights for use in certain indications, or the Collaboration License, (4) providing manufactured supply of preclinical and clinical ImmTOR, or the Manufactured Supply, (5) participation on identified steering committees responsible for the oversight of the collaboration, or the JSC Participation, and (6) granting an exclusive option to obtain a license under the IP Rights to research, develop and commercialize Licensed Products. The Company determined that the R&D Services, Research License, Collaboration License, Manufactured Supply, and JSC Participation were not capable of being distinct, and therefore must be combined into a single performance obligation. Therefore, promises (1) through (5) identified above were combined into a single performance obligation. Furthermore, the Company evaluated the Option Agreement and determined that it does not provide AskBio with a material right under ASC 606 as the option was not priced at a discount (see discussion of the option exercise in Note 12). The Company noted that AskBio did not meet the definition of a customer within the scope of ASC 606 for any distinct performance obligations as the Company concluded that such items were not an output of the Company’s ordinary activities. As such, the Company determined that the entire arrangement would be accounted for within the scope of ASC 808. In accordance with ASC 808, collaboration expenses are recognized within R&D expense and selling, general and administrative expense on the Company’s condensed consolidated statements of operations. Under certain collaborative arrangements, the Company is entitled to reimbursement of certain R&D expense. Activities under collaborative arrangements for which the Company is entitled to reimbursement are considered to be collaborative activities under the scope of ASC 808. For these units of account, the Company does not analogize to ASC 606 or recognize revenue. Rather, the Company analogizes to the guidance in ASC 730, which requires that reimbursements from counterparties be recognized as an offset to the related costs. In accordance with ASC 730, the Company records reimbursement payments received from collaborators as reductions to R&D expense. For the three and nine months ended September 30, 2021, the Company recognized $0.6 million and $2.3 million, respectively, of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. For the three and nine months ended September 30, 2020, the Company recognized $0.6 million and $2.6 million, respectively, of collaboration expense under the AskBio Collaboration Agreement. Massachusetts Institute of Technology In November 2008, the Company entered into an exclusive patent license agreement, or the MIT License, with the Massachusetts Institute of Technology, or MIT, under which the Company received an exclusive royalty-bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. In June 2020, the Company entered into a Fifth Amendment, or the MIT Amendment, to the MIT License, which is effective as of May 15, 2020. Pursuant to the MIT Amendment, certain of the Company’s diligence obligations were extended. The extension included the obligation to commence a Phase 3 trial for a licensed product by the second quarter of 2021 or to file an IND (or equivalent) with the FDA or comparable European regulatory agency for a licensed product by the second quarter of 2023. Additionally, certain of the Company’s development and regulatory milestones and payments upon achievement of such milestones were adjusted. As of September 30, 2021, and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the MIT License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The Company made no additional payments during the nine months ended September 30, 2021. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of September 30, 2021. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. On June 11, 2020, the Company entered into the Sobi License (see Note 12). In September 2020, Sobi paid the Company a one-time up-front payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. For income tax purposes, the transfer of trademark and product rights is treated as a sale and the net proceeds from the sale are taxed under the default installment method as cash is received by the Company. During the three months ended September 30, 2021, the Company completed an analysis of future tax obligations under the default installment sale method versus making a timely filed election on its 2020 tax return due October 15, 2021 to elect out of the installment sale method for income tax purposes. As a result, the Company elected out of the default installment sale treatment with the filing of its tax return. In the elect out method, the Company was taxed based upon the estimated fair value of all present and future proceeds from the sale and the Company utilized all of its available net operating losses and income tax credits, which served to reduce the federal and state tax liability. As such, the Company has recognized a total tax expense, inclusive of estimated penalties and interest, of $15.8 million as of September 30, 2021. As the Company recognizes future revenue under the Sobi license for US GAAP purposes, the Company will exclude that revenue from taxable income. The Company will maintain its full valuation allowance against its deferred tax assets for 2021, as the Company believes that it is more likely than not that the deferred tax assets will not be realized. Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 and 383 of the Internal Revenue Code due to ownership change limitations that have occurred previously, or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. As of December 31, 2020, the Company completed both a Section 382 and R&D tax credit study. As a result of using the elect out method described above, the Company utilized all available federal and state net operating losses and income tax credits with the filing of its tax returns for the year ended December 31, 2020. The statute of limitations for assessment by the United States Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan

Defined Contribution Plan	9 Months Ended
	Sep. 30, 2021
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution PlanThe Company maintains a defined contribution plan, or the 401(k) Plan, under Section 401(k) of the Internal Revenue Code. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaled less than $0.1 million during each of the three months ended September 30, 2021 and 2020, respectively, and $0.1 million during each of the nine months ended September 30, 2021 and 2020.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of September 30, 2021, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. On August 4, 2020, a putative stockholder of Selecta filed a stockholder derivative action, purportedly on behalf of Selecta and against certain current and former members of the Company’s Board of Directors, as well as one affiliated company owned by a current board member, in the Court of Chancery of the State of Delaware, namely Franchi v. Barabe, et al. The complaint alleges that the individual defendants breached their fiduciary duties and committed corporate waste when they authorized a private placement transaction, announced on December 19, 2019, at a price allegedly below fair value. The complaint further alleges that the four defendant directors who participated in the private placement were unjustly enriched in connection with the transaction. On September 25, 2020, the defendants filed a motion to dismiss the lawsuit. On November 6, 2020, the plaintiff filed an amended complaint, and the defendants filed a second motion to dismiss on January 8, 2021. On December 31, 2020, the Company received a litigation demand letter from two other putative stockholders relating to the same private placement transaction. On April 12, 2021, the Court of Chancery in the State of Delaware granted a motion to stay the litigation pending a review by a Special Committee appointed by the Company’s Board of Directors. At this time, the Company has not accrued a liability for this matter, as any liability has been determined to be either not estimable or not probable. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company’s business, financial position, results of operations or cash flows.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Licenses and Collaborations On October 25, 2021, the Company entered into a Collaboration and License Agreement, or the Ginkgo Agreement, with Ginkgo Bioworks Holdings, Inc., or Ginkgo. Under the Ginkgo Agreement, the Company will engage with Ginkgo to design novel and improved enzymes with transformative therapeutic potential to advance treatments for orphan and rare diseases. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments in the form of Selecta common stock, clinical and commercial milestone payments of up to $85.0 million in cash, as well as downstream value in the form of royalties on sales. On October 21, 2021, the Company entered into an Exclusive License Agreement, or the Genovis Agreement, with Genovis AB (publ.), or Genovis, a Swedish corporation. Under the Genovis Agreement, the Company paid to Genovis an upfront payment in exchange for an exclusive license to Genovis’ Xork IgG Protease enzyme technology across all therapeutic uses in humans, excluding research, preclinical, diagnostic and other potential non-therapeutic applications of the enzyme. Additionally, Genovis is eligible to earn development and sales-based milestones, as well as tiered royalties on worldwide sales in the low double digits. On October 1, 2021, the Company entered into a License Agreement, or the Takeda Agreement, with Takeda Pharmaceuticals USA, Inc., or Takeda. Under the Takeda Agreement, the Company granted Takeda an exclusive license to the Company’s ImmTOR technology initially for two specified disease indications within the field of lysosomal storage disorders. Under the terms of the Takeda Agreement, the Company is entitled to receive an upfront payment and up to $1.124 billion in future additional payments over the course of the partnership that are contingent on the achievement of development or commercial milestones or Takeda’s election to continue its activities at specified development stages. The Company is also eligible for tiered royalties on future commercial sales of any licensed products. “At-the-Market” Offerings 2021 Sales Agreement On October 25, 2021, the Company entered into a Sales Agreement, or the 2021 Sales Agreement, with SVB Leerink LLC to sell shares of the Company’s common stock, from time to time, through an “at the market” equity offering program under which SVB Leerink will act as sales agent. The shares of common stock sold pursuant to the 2021 Sales Agreement will be issued pursuant to the Company’s shelf registration statement on Form S-3 (File No. 333-241692), filed on August 6, 2020 with the Securities and Exchange Commission and related prospectus supplement, filed on October 25, 2021 with the Securities and Exchange Commission, for aggregate gross sales proceeds of up to $75.0 million. Under the 2021 Sales Agreement, the Company will set the parameters for the sale of shares, including the number of shares to be issued, the time period during which sales are requested to be made, limitations on the number of shares that may be sold in any one trading day and any minimum price below which sales may not be made. Subject to the terms and conditions of the Sales Agreement, SVB Leerink may sell the shares by methods deemed to be an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Act, in negotiated transactions at market prices prevailing at the time of sale or at prices related to such prevailing market prices and/or any other method permitted by law. If the Company and SVB Leerink so agree, SVB Leerink may act as principal in connection with the sale of shares under the 2021 Sales Agreement. The Company will pay SVB Leerink a commission of up to 3.0% of the gross sales proceeds of any shares of common stock sold through SVB Leerink under the 2021 Sales Agreement, and also has provided SVB Leerink with customary indemnification rights. The offering of shares of common stock pursuant to the 2021 Sales Agreement will terminate upon the earlier of (i) the sale of all shares of common stock subject to the 2021 Sales Agreement and (ii) the termination of the 2021 Sales Agreement as permitted therein. Warrants During October 2021, warrant holders exercised 1,642,036 common warrants on a cashless basis and received 1,076,669 shares of common stock.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In October 2020, the FASB issued ASU 2020-10, Codification Improvements , which updates various codification topics by clarifying or improving disclosure requirements to align with the SEC’s regulations. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) – Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. Not Yet Adopted In May 2021, the FASB issued ASU 2021-04, Earnings Per Share (Topic 260), Debt – Modifications and Extinguishments (Subtopic 470-50), Compensation – Stock Compensation (Topic 718), and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40): Issuer's Accounting for Certain Modifications or Exchanges of Freestanding Equity-Classified Written Call Options . ASU 2021-04 provides guidance as to how entities should account for a modification of the terms or conditions or an exchange of a freestanding equity-classified written call option (i.e., a warrant) that remains equity-classified after modification or exchange as an exchange of the original instrument for a new instrument. An entity should apply the guidance provided in ASU 2021-04 prospectively to modifications or exchanges occurring on or after the effective date. This new standard will be effective for us for fiscal years beginning after December 15, 2021 including interim periods within those fiscal years. The adoption of ASU 2021-04 is not expected to have an impact on the Company’s financial position or results of operations upon adoption. In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40) . ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. This new standard will be effective for us for fiscal years beginning after December 15, 2021, including interim periods within those fiscal years. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments . Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses . ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, and reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This new standard will be effective for us for fiscal years beginning after December 15, 2021. The adoption of ASU 2016-13 is not expected to have an impact on the Company’s financial position or results of operations upon adoption.

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Marketable Securities	The following table summarizes the marketable securities held as of September 30, 2021 (in thousands): Amortized Unrealized gains Unrealized losses Fair September 30, 2021 Corporate bonds $ 2,035 $ — $ (1) $ 2,034 Commercial paper 21,984 — — 21,984 Total $ 24,019 $ — $ (1) $ 24,018

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per-share data): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Numerator: Net loss $ (17,894) $ (9,729) $ (37,926) $ (53,430) Denominator: Weighted-average common shares outstanding - basic and diluted 115,169,949 105,325,788 113,161,622 98,968,359 Net loss per share: Basic and diluted $ (0.16) $ (0.09) $ (0.34) $ (0.54)
Schedule of Potential Common Shares Issuable Upon Conversion of Warrants	The following table represents the potential dilutive common shares excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Options, RSUs and ESPP shares 11,701,844 7,638,839 11,701,844 7,638,839 Warrants to purchase common stock 12,378,016 13,888,525 12,378,016 13,888,525 Total 24,079,860 21,527,364 24,079,860 21,527,364

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Assets Measured at Fair Value on a Recurring Basis	The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash equivalents) $ 56,563 $ 56,563 $ — $ — Marketable securities: Corporate bonds 2,034 — 2,034 — Commercial paper 21,984 — 21,984 — Total assets $ 80,581 $ 56,563 $ 24,018 $ — Liabilities: Warrant liabilities $ 40,043 $ — $ — $ 40,043 Total liabilities $ 40,043 $ — $ — $ 40,043 December 31, 2020 Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash equivalents) $ 80,576 $ 80,576 $ — $ — Total assets $ 80,576 $ 80,576 $ — $ — Liabilities: Warrant liabilities $ 28,708 $ — $ — $ 28,708 Total liabilities $ 28,708 $ — $ — $ 28,708
Schedule of Cash and Cash Equivalents	The Company’s consolidated statement of cash flows includes the following as of September 30, 2021 and 2020 (in thousands): September 30, 2021 2020 Cash and cash equivalents $ 114,645 $ 146,261 Long-term restricted cash 1,379 1,379 Total cash, cash equivalents, and restricted cash $ 116,024 $ 147,640
Schedule of Restricted Cash and Cash Equivalents	The Company’s consolidated statement of cash flows includes the following as of September 30, 2021 and 2020 (in thousands): September 30, 2021 2020 Cash and cash equivalents $ 114,645 $ 146,261 Long-term restricted cash 1,379 1,379 Total cash, cash equivalents, and restricted cash $ 116,024 $ 147,640
Schedule of Fair Value Measurement Inputs and Valuation Techniques	A summary of the Black-Scholes pricing model assumptions used to record the fair value of the warrant liability is as follows: September 30, 2021 Risk-free interest rate 0.53 % Dividend yield — Expected life (in years) 3.23 Expected volatility 98.07 %
Schedule Of Changes In The Warrant Liabilities	The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 10), for the nine months ended September 30, 2021 (in thousands): Warrant liabilities Fair value as of December 31, 2020 $ 28,708 Change in fair value 11,335 Fair value as of September 30, 2021 $ 40,043

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2021
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): September 30, December 31, 2021 2020 Laboratory equipment $ 5,095 $ 4,427 Computer equipment and software 734 532 Leasehold improvements 45 38 Furniture and fixtures 327 327 Office equipment 163 163 Construction in process 93 163 Total property and equipment 6,457 5,650 Less accumulated depreciation (4,650) (4,255) Property and equipment, net $ 1,807 $ 1,395

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): September 30, December 31, 2021 2020 Payroll and employee related expenses $ 2,552 $ 3,049 Collaboration and licensing 1,350 1,350 Accrued patent fees 457 534 Accrued external research and development costs 4,551 2,029 Accrued professional and consulting services 1,205 798 Accrued interest 165 170 Other 462 216 Accrued expenses $ 10,742 $ 8,146

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Schedule of Lease, Cost	For the three and nine months ended September 30, 2021 and 2020 the components of lease costs were as follows (in thousands): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Operating lease cost $ 457 $ 506 $ 1,355 $ 1,590 Variable lease cost 182 123 652 496 Short-term lease cost 2 3 7 8 Total lease cost $ 641 $ 632 $ 2,014 $ 2,094 The following summarizes additional information related to operating leases: September 30, 2021 2020 Weighted-average remaining lease term 6.6 years 7.7 years Weighted-average discount rate 8.9 % 8.9 %
Schedule of Lessee, Operating Lease, Liability, Maturity	The maturity of the Company’s operating lease liabilities as of September 30, 2021 were as follows (in thousands): September 30, 2021 2021 (remainder) $ 457 2022 1,866 2023 1,922 2024 1,980 2025 2,039 Thereafter 4,945 Total future minimum lease payments 13,209 Less imputed interest 3,323 Total operating lease liabilities $ 9,886
Schedule of Operating Lease, Lease Income	The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): September 30, 2021 2020 Cash paid for amounts included in the measurement of lease liabilities: $ 1,355 $ 2,079

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments on the Term Loans	Future minimum principal and interest payments on the 2020 Term Loan as of September 30, 2021 are as follows (in thousands): 2021 (remainder) $ 499 2022 7,343 2023 8,611 2024 8,027 2025 7,274 Total minimum debt payments 31,754 Less: Amount representing interest (4,505) Less: Debt discount and deferred charges (1,820) Less: Current portion of loan payable (4,125) Loan payable, net of current portion $ 21,304

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Schedule of Stockholders' Equity Note, Warrants or Rights	Number of Warrants Equity Liability classified Total Weighted average Outstanding at September 30, 2021 292,469 12,085,547 12,378,016 $ 1.60
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as follows: As of September 30, 2021 December 31, 2020 Exercise of common warrants 12,378,016 12,378,016 Shares available for future stock incentive awards 6,049,422 4,916,374 Unvested restricted stock units 561,888 87,500 Outstanding common stock options 11,082,277 7,775,249 Total 30,071,603 25,157,139

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Schedule of Stock-Based Compensation	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands): Three Months Ended Nine Months Ended 2021 2020 2021 2020 Research and development $ 791 $ 532 $ 2,331 $ 1,773 General and administrative 1,113 764 3,136 2,413 Total stock-based compensation expense $ 1,904 $ 1,296 $ 5,467 $ 4,186
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Three Months Ended Nine Months Ended 2021 2020 2021 2020 Risk-free interest rate 1.01 % 0.24 % 0.79 % 1.19 % Dividend yield — — — — Expected term 6.06 6.08 6.03 6.05 Expected volatility 93.55 % 94.88 % 95.05 % 90.19 % Weighted-average fair value of common stock $ 4.19 $ 2.47 $ 3.58 $ 2.39
Summary of Options Activity	The following table summarizes the stock option activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan: Weighted-average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employees Outstanding at December 31, 2020 7,302,176 $ 3.98 8.43 $ 4,456 Granted 4,609,811 $ 3.58 Exercised (397,492) $ 1.72 Forfeited (905,291) $ 2.82 Outstanding at September 30, 2021 10,609,204 $ 3.99 8.44 $ 11,080 Vested at September 30, 2021 3,583,316 $ 5.27 7.32 $ 4,133 Vested and expected to vest at September 30, 2021 9,775,480 $ 4.04 8.36 $ 10,400 Non-employee consultants Outstanding at December 31, 2020 473,073 $ 5.89 5.23 $ 86 Granted — $ — Exercised — $ — Forfeited — $ — Outstanding at September 30, 2021 473,073 $ 5.89 4.48 $ 282 Vested at September 30, 2021 473,073 $ 5.89 4.48 $ 282 Vested and expected to vest at September 30, 2021 473,073 $ 5.89 4.48 $ 282
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan: Number of shares Weighted average Unvested at December 31, 2020 87,500 $ 6.03 Granted 603,600 3.24 Vested (32,812) 6.03 Forfeited (96,400) 2.99 Unvested at September 30, 2021 561,888 $ 3.56

Revenue Arrangements (Tables)

Revenue Arrangements (Tables)	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customer [Abstract]
Schedule of Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the nine months ended September 30, 2021 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Nine Months Ended September 30, 2021 Contract liabilities: Deferred revenue $ 110,796 $ 3,000 $ (31,424) $ 82,372 Total contract liabilities $ 110,796 $ 3,000 $ (31,424) $ 82,372

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, restricted cash and marketable securities	$ 140,000
Restricted cash and cash equivalents	1,400
Accumulated deficit	(442,555)	$ (404,629)
Russian subsidiary
Cash and Cash Equivalents [Line Items]
Cash maintained in Russian bank accounts	$ 300

Marketable Securities - Summary

Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 24,019,000
Unrealized gains	0
Unrealized losses	(1,000)
Fair value	24,018,000	$ 0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,035,000
Unrealized gains	0
Unrealized losses	(1,000)
Fair value	2,034,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	21,984,000
Unrealized gains	0
Unrealized losses	0
Fair value	$ 21,984,000

Marketable Securities - Narrati

Marketable Securities - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Marketable securities adjusted for other than temporary declines in fair value	$ 0
Marketable securities:	$ 24,018,000	$ 0

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (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	$ (17,894)	$ 4,565	$ (24,597)	$ (9,729)	$ (24,081)	$ (19,620)	$ (37,926)	$ (53,430)
Denominator:
Weighted-average common shares outstanding, basic (in shares)	115,169,949			105,325,788			113,161,622	98,968,359
Weighted‑average common shares outstanding—diluted (in shares)	115,169,949			105,325,788			113,161,622	98,968,359
Net loss per share:
Basic (in dollars per share)	$ (0.16)			$ (0.09)			$ (0.34)	$ (0.54)
Diluted (in dollars per share)	$ (0.16)			$ (0.09)			$ (0.34)	$ (0.54)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Potential common shares
Total (in shares)	24,079,860	21,527,364	24,079,860	21,527,364
Shares available for future stock incentive awards
Potential common shares
Total (in shares)	11,701,844	7,638,839	11,701,844	7,638,839
Warrants to purchase common stock
Potential common shares
Total (in shares)	12,378,016	13,888,525	12,378,016	13,888,525

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Assets:
Marketable securities:	$ 24,018,000	$ 0
Liabilities:
Warrant liabilities	40,043,000	28,708,000
Corporate bonds
Assets:
Marketable securities:	2,034,000
Commercial paper
Assets:
Marketable securities:	21,984,000
Recurring
Assets:
Total assets	80,581,000	80,576,000
Liabilities:
Warrant liabilities	40,043,000	28,708,000
Total liabilities	40,043,000	28,708,000
Recurring | Level 1
Assets:
Total assets	56,563,000	80,576,000
Liabilities:
Warrant liabilities	0	0
Total liabilities	0	0
Recurring | Level 2
Assets:
Total assets	24,018,000	0
Liabilities:
Warrant liabilities	0	0
Total liabilities	0	0
Recurring | Level 3
Assets:
Total assets	0	0
Liabilities:
Warrant liabilities	40,043,000	28,708,000
Total liabilities	40,043,000	28,708,000
Recurring | Corporate bonds
Assets:
Marketable securities:	2,034,000
Recurring | Corporate bonds | Level 1
Assets:
Marketable securities:	0
Recurring | Corporate bonds | Level 2
Assets:
Marketable securities:	2,034,000
Recurring | Corporate bonds | Level 3
Assets:
Marketable securities:	0
Recurring | Commercial paper
Assets:
Marketable securities:	21,984,000
Recurring | Commercial paper | Level 1
Assets:
Marketable securities:	0
Recurring | Commercial paper | Level 2
Assets:
Marketable securities:	21,984,000
Recurring | Commercial paper | Level 3
Assets:
Marketable securities:	0
Recurring | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	56,563,000	80,576,000
Recurring | Money market funds (included in cash equivalents) | Level 1
Assets:
Money market funds (included in cash equivalents)	56,563,000	80,576,000
Recurring | Money market funds (included in cash equivalents) | Level 2
Assets:
Money market funds (included in cash equivalents)	0	0
Recurring | Money market funds (included in cash equivalents) | Level 3
Assets:
Money market funds (included in cash equivalents)	$ 0	$ 0

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 114,645	$ 138,685	$ 146,261
Long-term restricted cash	1,379	1,379	1,379
Total cash, cash equivalents, and restricted cash	$ 116,024	$ 140,064	$ 147,640	$ 91,551

Fair Value Measurements - Assum

Fair Value Measurements - Assumptions used to record the fair value of the warrants (Details) - Valuation technique, option pricing model	Sep. 30, 2021
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0053
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0
Expected life (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	3 years 2 months 23 days
Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.9807

Fair Value Measurements - Chang

Fair Value Measurements - Change in the warrant liabilities (Details) - Warrant liabilities $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value as of December 31, 2020	$ 28,708
Change in fair value	11,335
Fair value as of September 30, 2021	$ 40,043

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net	$ 0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability, transfers, net		$ 0

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 6,457	$ 5,650
Less accumulated depreciation	(4,650)	(4,255)
Property and equipment, net	1,807	1,395
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	5,095	4,427
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	734	532
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	45	38
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	327	327
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	163	163
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 93	$ 163

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.1	$ 0.1	$ 0.4	$ 0.5

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 2,552	$ 3,049
Collaboration and licensing	1,350	1,350
Accrued patent fees	457	534
Accrued external research and development costs	4,551	2,029
Accrued professional and consulting services	1,205	798
Accrued interest	165	170
Other	462	216
Accrued expenses	$ 10,742	$ 8,146

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2019USD ($)ft²	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Operating Leased Assets [Line Items]
Lessee-paid construction costs		$ 641	$ 632	$ 2,014	$ 2,094
Operating lease, weighted average discount rate, percent		8.90%	8.90%	8.90%	8.90%
Letter of credit
Operating Leased Assets [Line Items]
Long-term restricted cash		$ 1,400		$ 1,400		$ 1,400
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Area of additional office space leased | ft²	25,078
Lessee-paid construction costs	$ 800
Term of contract	8 years

Leases - Components of lease co

Leases - Components of lease costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Operating lease cost	$ 457	$ 506	$ 1,355	$ 1,590
Variable lease cost	182	123	652	496
Short-term lease cost	2	3	7	8
Total lease cost	$ 641	$ 632	$ 2,014	$ 2,094

Leases - Operating Lease, Liabi

Leases - Operating Lease, Liability, Maturity (Details) $ in Thousands	Sep. 30, 2021USD ($)
Leases [Abstract]
2021 (remainder)	$ 457
2022	1,866
2023	1,922
2024	1,980
2025	2,039
Thereafter	4,945
Total future minimum lease payments	13,209
Less imputed interest	3,323
Lease liabilities	$ 9,886

Leases - Other Information (Det

Leases - Other Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 1,355	$ 2,079

Leases - Lease Term and Discoun

Leases - Lease Term and Discount Rate (Details)	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Operating lease, weighted average remaining lease term	6 years 7 months 6 days	7 years 8 months 12 days
Operating lease, weighted average discount rate, percent	8.90%	8.90%

Debt - Term Loans (Narrative) (

Debt - Term Loans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	Aug. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Line of Credit Facility [Line Items]
Final payment fee		$ 2,300		$ 2,300
Class of warrant or right, number of securities called by warrants or rights (in shares)	196,850
Exercise price (in dollars per share)	$ 2.54	$ 1.60		$ 1.60
Class of warrant or right, expiration period	10 years
Issuance of common warrants with long-term debt, net	$ 400		$ 444
Loss on extinguishment of debt		$ 0	461	$ 0	$ 461
2020 Term Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	35,000
Debt instrument, additional accrued interest in the event of default				0.050
Debt instrument, redemption price, percentage of principal amount redeemed				9.00%
Debt instrument, default, bankruptcy, or insolvency value of triggering accelerated redemption	500
Debt issuance costs, line of credit arrangements (less than)	400
Term loan facility		$ 25,000		$ 25,000		$ 25,000
2020 Term Loans | Maximum
Line of Credit Facility [Line Items]
Debt instrument, prepayment fee, percentage				0.030
2020 Term Loans | Minimum
Line of Credit Facility [Line Items]
Debt instrument, prepayment fee, percentage				0.010
2020 Term Loans | Debt Instrument, Redemption, Period One
Line of Credit Facility [Line Items]
Basis spread on variable rate				7.90%
2020 Term Loans | Debt Instrument, Redemption, Period Two
Line of Credit Facility [Line Items]
Basis spread on variable rate				4.65%
2020 Term Loans | Debt Instrument, Redemption, Period Three
Line of Credit Facility [Line Items]
Basis spread on variable rate				10.00%
Term A Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	25,000
Term B Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	10,000
Term B Loans | Warrants to purchase common stock
Line of Credit Facility [Line Items]
Sale common stock	200,000
2017 Term Loans
Line of Credit Facility [Line Items]
Repayments of lines of credit	$ 13,700
Loss on extinguishment of debt			$ 500		$ 500

Debt - Schedule of Future Minim

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Sep. 30, 2021USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2021 (remainder)	$ 499
2022	7,343
2023	8,611
2024	8,027
2025	7,274
Total minimum debt payments	31,754
Less: Amount representing interest	(4,505)
Less: Debt discount and deferred charges	(1,820)
Less: Current portion of loan payable	(4,125)
Loan payable, net of current portion	$ 21,304

Equity - Narrative (Details)

Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands

Aug. 06, 2020

Jul. 28, 2020

Jun. 11, 2020

Dec. 18, 2019

Aug. 31, 2017

Sep. 30, 2021

Sep. 30, 2020

Mar. 31, 2020

Sep. 30, 2021

Sep. 30, 2020

Dec. 31, 2020

Aug. 31, 2020

Subsidiary, Sale of Stock [Line Items]

Shares issued, value

$ 30,906

$ 2,137

Class of warrant or right, outstanding (in shares)

12,378,016

12,378,016

Exercise price (in dollars per share)

$ 1.60

$ 1.60

$ 2.54

Warrant liabilities

$ 40,043

$ 40,043

$ 28,708

August 2020 Shelf Registration Statement

Subsidiary, Sale of Stock [Line Items]

Sale common stock

$ 200,000

At-The-Market Offering

Subsidiary, Sale of Stock [Line Items]

Sale common stock

$ 50,000

Sale of stock, number of shares issued in transaction (in shares)

6,883,153

1,069,486

Share price (in dollars per share)

$ 2.16

Shares issued, value

$ 30,900

$ 2,100

2020 Sale Agreement - At-the-Market Offering

Subsidiary, Sale of Stock [Line Items]

Sale common stock

$ 50,000

SOBI Purchase Agreement

Subsidiary, Sale of Stock [Line Items]

Sale common stock

$ 25,000

Sale of stock, number of shares issued in transaction (in shares)

5,416,390

Share price (in dollars per share)

$ 4.6156

Sale of stock, percentage of ten-day volume weighted average price of common stock

120.00%

Lock-up from closing, term

1 year

Redemption premium

$ 14,500

December 2019 Financing

Subsidiary, Sale of Stock [Line Items]

Sale common stock

$ 65,600

Sale of stock, number of shares issued in transaction (in shares)

37,634,883

Share price (in dollars per share)

$ 1.46

Class of warrant or right, outstanding (in shares)

22,988,501

Warrant issued

$ 40,700

Issuance costs

$ 4,400

Warrant liabilities

40,000

40,000

Warrant, increase (decrease) in equity, amount

$ (600)

$ (4,800)

$ 11,300

$ 3,600

December 2019 Financing | Pre-Funded Warrant

Subsidiary, Sale of Stock [Line Items]

Class of warrant or right, number of warrants exercised (in shares)

8,342,128

Exercise price (in dollars per share)

$ 0.0001

December 2019 Financing | Common Warrant

Subsidiary, Sale of Stock [Line Items]

Share price (in dollars per share)

0.125

Exercise price (in dollars per share)

$ 1.46

Warrants and rights outstanding, term

5 years

Equity - Warranty Activity (Det

Equity - Warranty Activity (Details) - $ / shares	Sep. 30, 2021	Aug. 31, 2020
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	12,378,016
Exercise price (in dollars per share)	$ 1.60	$ 2.54
Exercise of common warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	292,469
Exercise of common warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	12,085,547

Equity - Schedule of Authorized

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Total (in shares)	30,071,603	25,157,139
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	6,049,422	4,916,374
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	561,888	87,500
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	11,082,277	7,775,249
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	12,378,016	12,378,016

Stock Incentive Plans - Narrati

Stock Incentive Plans - Narrative (Details)	1 Months Ended			3 Months Ended			9 Months Ended
	Sep. 30, 2021USD ($)shares	Jan. 31, 2021shares	Jan. 31, 2020shares	Sep. 30, 2021USD ($)$ / sharesshares	Mar. 31, 2021USD ($)$ / sharesshares	Sep. 30, 2020USD ($)$ / shares	Sep. 30, 2021USD ($)installment$ / sharesshares	Sep. 30, 2020USD ($)$ / shares	Dec. 31, 2020shares	Mar. 31, 2019shares	Sep. 30, 2018shares	Jun. 30, 2016shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	30,071,603			30,071,603			30,071,603		25,157,139
Stock‑based compensation expense | $				$ 1,904,000		$ 1,296,000	$ 5,467,000	$ 4,186,000
Unrecognized compensation expense related to unvested non-employee stock options | $	$ 0			$ 0			$ 0
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	6,049,422			6,049,422			6,049,422		4,916,374
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares				$ 3.17		$ 2.73	$ 1.88	$ 1.77
Unrecognized compensation expense related to unvested employee stock options | $	$ 12,800,000			$ 12,800,000			$ 12,800,000
Weighted average period for recognition							2 years 10 months 24 days
Estimated forfeitures rate					10.00%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)							603,600
Reserved for issuance (in dollars per share) | $ / shares					$ 2.99
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)												1,210,256
Number of shares authorized, increase (in shares)		4,322,850	3,453,022
Common stock authorized and reserved for future issuance (in shares)	1,935,395			1,935,395			1,935,395
2016 Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)	36,300	369,800			98,750
Award vesting term	4 years						4 years
Awarded in period (in shares)					197,500
Reserved for issuance (in shares)					98,750
Number of award vesting periods | installment							2
Vested in period | $					$ 1,400,000
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)	1,591,661			1,591,661			1,591,661
Common stock authorized and reserved for future issuance (in shares)										2,000,000	1,175,000
Employment Inducement Incentive Award Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognition							2 years 1 month 6 days
Unrecognized compensation expense | $	$ 1,400,000			$ 1,400,000			$ 1,400,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized, increase (in shares)		1,080,711	863,254
Common stock authorized and reserved for future issuance (in shares)	2,522,366			2,522,366			2,522,366					173,076
Stock‑based compensation expense | $				$ 100,000		$ 100,000	$ 100,000	$ 100,000

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,904	$ 1,296	$ 5,467	$ 4,186
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	791	532	2,331	1,773
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,113	$ 764	$ 3,136	$ 2,413

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Assumptions (Details) - Shares available for future stock incentive awards - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.01%	0.24%	0.79%	1.19%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term	6 years 21 days	6 years 29 days	6 years 10 days	6 years 18 days
Expected volatility	93.55%	94.88%	95.05%	90.19%
Weighted-average fair value of common stock (in dollars per share)	$ 4.19	$ 2.47	$ 3.58	$ 2.39

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Options Activity (Details) - 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Employees
Number of Options
Beginning balance (in shares) | shares	7,302,176
Granted (in shares) | shares	4,609,811
Exercised (in shares) | shares	(397,492)
Forfeited (in shares) | shares	(905,291)
Ending balance (in shares) | shares	10,609,204	7,302,176
Vested (in shares) | shares	3,583,316
Vested and expected to vest (in shares) | shares	9,775,480
Weighted-average exercise price
Beginning balance (in dollars per share) | $ / shares	$ 3.98
Granted (in dollars per share) | $ / shares	3.58
Exercised (in dollars per share) | $ / shares	1.72
Forfeited (in dollars per share) | $ / shares	2.82
Ending balance (in dollars per share) | $ / shares	3.99	$ 3.98
Vested (in dollars per share) | $ / shares	5.27
Vested and expected to vest (in dollars per share) | $ / shares	$ 4.04
Weighted-average remaining contractual term
Outstanding, term	8 years 5 months 8 days	8 years 5 months 4 days
Vested, term	7 years 3 months 25 days
Vested and expected to vest, term	8 years 4 months 9 days
Aggregate intrinsic value
Outstanding | $	$ 11,080	$ 4,456
Vested | $	4,133
Vested and expected to vest | $	$ 10,400
Non-employee consultants
Number of Options
Beginning balance (in shares) | shares	473,073
Granted (in shares) | shares	0
Exercised (in shares) | shares	0
Forfeited (in shares) | shares	0
Ending balance (in shares) | shares	473,073	473,073
Vested (in shares) | shares	473,073
Vested and expected to vest (in shares) | shares	473,073
Weighted-average exercise price
Beginning balance (in dollars per share) | $ / shares	$ 5.89
Granted (in dollars per share) | $ / shares	0
Exercised (in dollars per share) | $ / shares	0
Forfeited (in dollars per share) | $ / shares	0
Ending balance (in dollars per share) | $ / shares	5.89	$ 5.89
Vested (in dollars per share) | $ / shares	5.89
Vested and expected to vest (in dollars per share) | $ / shares	$ 5.89
Weighted-average remaining contractual term
Outstanding, term	4 years 5 months 23 days	5 years 2 months 23 days
Vested, term	4 years 5 months 23 days
Vested and expected to vest, term	4 years 5 months 23 days
Aggregate intrinsic value
Outstanding | $	$ 282	$ 86
Vested | $	282
Vested and expected to vest | $	$ 282

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock Units (Details) - Restricted Stock Units (RSUs)	9 Months Ended
	Sep. 30, 2021$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested at beginning of period (in shares) | shares	87,500
Grants in period (in shares) | shares	603,600
Vested in period (in shares) | shares	(32,812)
Forfeited in period (in shares) | shares	(96,400)
Unvested at end of period (in shares) | shares	561,888
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested at beginning of period (in dollars per share) | $ / shares	$ 6.03
Grants in period (in dollars per share) | $ / shares	3.24
Vested in period (in dollars per share) | $ / shares	6.03
Forfeited in period (in dollars per share) | $ / shares	2.99
Unvested at end of period (in dollars per share) | $ / shares	$ 3.56

Stock Incentive Plans - Employe

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2021	Jan. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			30,071,603		30,071,603		25,157,139
Stock-based compensation expense			$ 1,904	$ 1,296	$ 5,467	$ 4,186
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			2,522,366		2,522,366			173,076
Number of shares authorized, increase (in shares)	1,080,711	863,254
Shares of common stock issued to employees under the ESPP (in shares)					58,794
Stock-based compensation expense			$ 100	$ 100	$ 100	$ 100

Revenue Arrangements - Narrativ

Revenue Arrangements - Narrative (Details)	Jul. 28, 2020USD ($)	Jun. 13, 2020USD ($)	Jun. 11, 2020USD ($)obligation	Dec. 17, 2019USD ($)	Sep. 30, 2021USD ($)	Jun. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)amendment
Disaggregation of Revenue [Line Items]
Short-term contract liability					$ 62,315,000			$ 62,315,000		$ 72,050,000
Long-term contract liability					20,057,000			20,057,000		38,746,000
Contract assets amortization (less than)								400,000
Accounts receivable					7,324,000			7,324,000		7,224,000
Revenue recognized								31,424,000
Remaining performance obligation					82,400,000			82,400,000
SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Sale common stock			$ 25,000,000
Redemption premium	$ 14,500,000
Short-term contract liability					55,900,000			55,900,000		68,300,000
Long-term contract liability					5,500,000			5,500,000		24,200,000
Contract assets					1,000,000			1,000,000
Accounts receivable					7,000,000			7,000,000		6,900,000
Revenue recognized					24,300,000		$ 4,300,000	54,800,000	$ 4,300,000
SOBI Purchase Agreement | Prepaid Expenses and Other Current Assets
Disaggregation of Revenue [Line Items]
Contract assets					900,000			900,000
SOBI Purchase Agreement | Other Current Assets
Disaggregation of Revenue [Line Items]
Contract assets					100,000			100,000
Collaborative Arrangement
Disaggregation of Revenue [Line Items]
Upfront cash payment			75,000,000
Sales milestone payments			$ 630,000,000
License and option agreement, written day notice period before cancellation			180 days
Period after first commercial sale when the Company is eligible to receive royalties			10 years
Number of obligations | obligation			3
Development milestone			$ 5,000,000
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Upfront cash payment		$ 2,000,000
Sales milestone payments						$ 3,000,000
License and option agreement, written day notice period before cancellation		30 days
Short-term contract liability					4,800,000			4,800,000		$ 2,000,000
Revenue recognized					200,000		300,000	200,000	300,000
Option term		24 months
Number of amendments | amendment										2
License Agreement For Pompe Disease
Disaggregation of Revenue [Line Items]
Upfront cash payment				$ 7,000,000
Sales milestone payments				$ 237,000,000
Period after first commercial sale when the Company is eligible to receive royalties				10 years
Short-term contract liability					1,700,000			1,700,000
Long-term contract liability										$ 5,300,000
Revenue recognized					$ 0		$ 0	0	$ 0
Initial up-front cash payment				$ 2,000,000
Initial upfront execution fee payment				$ 5,000,000
Skolkovo
Disaggregation of Revenue [Line Items]
Revenue recognized								$ 100,000

Revenue Arrangements - Schedule

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Contract liabilities:
Deferred revenue, beginning of period	$ 110,796
Contract liabilities, beginning of period	110,796
Deferred revenue, additions	3,000
Contract liabilities, additions	3,000
Deferred revenue, deductions	(31,424)
Contract liabilities, deductions	(31,424)
Deferred revenue, end of period	82,372
Contract liabilities, end of period	$ 82,372

Related-Party Transactions - Na

Related-Party Transactions - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Founders
Related Party Transaction [Line Items]
Consulting services expenses (less than)	$ 0.1	$ 0.1	$ 0.1	$ 0.1

Collaboration and License Agr_2

Collaboration and License Agreements - Cyrus Biotechnology, Inc. (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 07, 2021	Sep. 30, 2021	Dec. 31, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities without readily determinable fair value, amount		$ 2,000	$ 0
Cyrus Biotechnology, Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 1,500,000
Equity securities, stock purchase agreement, shares purchased (in shares)	2,326,934
Equity securities without readily determinable fair value, amount	$ 2,000
Cyrus Biotechnology, Inc. | Series B Preferred Stock Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities, stock purchase agreement, par value per share (in dollars per share)	$ 0.0001
Equity securities, stock purchase agreement, purchase price per share (in dollars per share)	$ 0.8595

Collaboration and License Agr_3

Collaboration and License Agreements - AskBio (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Operating expenses	$ 26,396	$ 18,380	$ 63,815	$ 53,569
AskBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Operating expenses	$ 600	$ 600	$ 2,300	$ 2,600
Cost share percentage			50.00%

Collaboration and License Agr_4

Collaboration and License Agreements - MIT (Narrative) (Details) - Spark License Agreement	9 Months Ended
	Sep. 30, 2021USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	$ 0
MIT
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement	$ 400,000

Collaboration and License Agr_5

Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 7
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 15

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	Jun. 11, 2020	Sep. 30, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total tax expense, inclusive of estimated penalties and interest		$ 15.8
Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment	$ 75
Sales milestone payments	$ 630

Defined Contribution Plan - Nar

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Retirement Benefits [Abstract]
Vesting period			4 years
Employer contribution made	$ 0.1	$ 0.1	$ 0.1	$ 0.1

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ in Millions	Oct. 25, 2021	Oct. 01, 2021	Oct. 31, 2021	Aug. 31, 2017
At-The-Market Offering
Subsequent Event [Line Items]
Sale common stock				$ 50
Subsequent Event
Subsequent Event [Line Items]
Class of warrant or right, number of warrants exercised (in shares)			1,642,036
Issuance of common stock upon exercise of common warrants (in shares)			1,076,669
Subsequent Event | At-The-Market Offering
Subsequent Event [Line Items]
Sale common stock	$ 75
Percentage of commission by gross sales proceeds of any shares of common stock sold	3.00%
Subsequent Event | Ginkgo Agreement
Subsequent Event [Line Items]
Clinical and commercial milestone payment, expected	$ 85
Subsequent Event | Takeda Agreement
Subsequent Event [Line Items]
Future additional payments, expected		$ 1,124