Acceleron Pharma (XLRN) 10-Q 4 Nov 21 2021 Q3 Quarterly report Financial data

Cover

Cover - shares	9 Months Ended
	Sep. 30, 2021	Nov. 01, 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	001-36065
Entity Registrant Name	ACCELERON PHARMA INC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-0072226
Entity Address, Address Line One	128 Sidney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	649-9200
Title of 12(b) Security	Common Stock, $0.001 per share
Trading Symbol	XLRN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		61,155,826
Entity Central Index Key	0001280600
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 336,456	$ 670,952
Short-term investments	254,113	178,951
Collaboration receivables (all amounts are with a related party)	34,195	26,101
Prepaid expenses and other current assets	71,844	17,265
Total current assets	696,608	893,269
Property and equipment, net	9,435	7,768
Operating lease - right of use asset, net	18,310	21,988
Long-term investments	61,950	7,585
Other assets	1,545	1,727
Total assets	787,848	932,337
Current liabilities:
Accounts payable	5,086	8,472
Accrued expenses	44,122	44,681
Operating lease liability - right of use	8,163	7,010
Total current liabilities	57,371	60,163
Operating lease liability - right of use, net of current portion	11,951	17,067
Other non-current liabilities	373	0
Total liabilities	69,695	77,230
Commitments and contingencies
Stockholders’ equity:
Undesignated preferred stock, $0.001 par value: 25,000,000 shares authorized and no shares issued or outstanding	0	0
Common stock, $0.001 par value: 175,000,000 shares authorized; 61,130,687 and 60,383,867 shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively	61	60
Additional paid-in capital	1,793,384	1,732,772
Accumulated deficit	(1,074,894)	(877,437)
Accumulated other comprehensive loss	(398)	(288)
Total stockholders’ equity	718,153	855,107
Total liabilities and stockholders’ equity	$ 787,848	$ 932,337

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	175,000,000	175,000,000
Common stock, shares issued (in shares)	61,130,687	60,383,867
Common stock, shares outstanding (in shares)	61,130,687	60,383,867

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaboration revenue:
Revenues	$ 34,199	$ 22,561	$ 86,897	$ 66,656
Costs and expenses:
Research and development	59,717	40,747	173,146	116,663
Selling, general and administrative	45,112	21,042	111,646	59,705
Total costs and expenses	104,829	61,789	284,792	176,368
Loss from operations	(70,630)	(39,228)	(197,895)	(109,712)
Other income (expense), net	180	(6)	466	1,108
Loss before income taxes	(70,450)	(39,234)	(197,429)	(108,604)
Income tax provision	(15)	(11)	(28)	(31)
Net loss	(70,465)	(39,245)	(197,457)	(108,635)
Other comprehensive income (loss):
Net unrealized holding gains (losses) on short-term and long-term investments during the period, net of tax	11	(142)	(110)	(248)
Comprehensive loss	$ (70,454)	$ (39,387)	$ (197,567)	$ (108,883)
Net loss per share- basic (in dollars per share)	$ (1.16)	$ (0.66)	$ (3.25)	$ (1.95)
Net loss per share- diluted (in dollars per share)	$ (1.16)	$ (0.66)	$ (3.25)	$ (1.95)
Weighted-average number of common shares used in computing net loss per share- basic (in shares)	60,937	59,640	60,748	55,635
Weighted-average number of common shares used in computing net loss per share- diluted (in shares)	60,937	59,640	60,748	55,635
Milestone
Collaboration revenue:
Revenues	$ 0	$ 0	$ 0	$ 25,000
Cost-sharing, net
Collaboration revenue:
Revenues	2,156	3,298	6,812	9,800
Royalty
Collaboration revenue:
Revenues	$ 32,043	$ 19,263	$ 80,085	$ 31,856

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2019		53,123,567
Stockholders' equity beginning balance at Dec. 31, 2019	$ 449,476	$ 53	$ 1,160,807	$ (711,407)	$ 23
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	6,679		6,679
Exercise of stock options (in shares)		295,757
Exercise of stock options	8,485		8,485
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		77,949
Vesting of restricted stock units, net of shares withheld for taxes	(472)		(472)
Issuance of common stock related to ESPP (in shares)		22,647
Issuance of common stock related to ESPP	860		860
Unrealized loss on available-for-sale securities, net of tax	(267)				(267)
Net loss	(50,939)			(50,939)
Stockholders' equity ending balance (in shares) at Mar. 31, 2020		53,519,920
Stockholders' equity ending balance at Mar. 31, 2020	413,822	$ 53	1,176,359	(762,346)	(244)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2019		53,123,567
Stockholders' equity beginning balance at Dec. 31, 2019	449,476	$ 53	1,160,807	(711,407)	23
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized loss on available-for-sale securities, net of tax	(248)
Net loss	(108,635)
Stockholders' equity ending balance (in shares) at Sep. 30, 2020		60,052,046
Stockholders' equity ending balance at Sep. 30, 2020	$ 895,136	$ 60	1,715,343	(820,042)	(225)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in dollars per share)	$ 0.001
Stockholders' equity beginning balance (in shares) at Mar. 31, 2020		53,519,920
Stockholders' equity beginning balance at Mar. 31, 2020	$ 413,822	$ 53	1,176,359	(762,346)	(244)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	7,140		7,140
Exercise of stock options (in shares)		617,441
Exercise of stock options	19,610	$ 1	19,609
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		29,351
Vesting of restricted stock units, net of shares withheld for taxes	(663)		(663)
Unrealized loss on available-for-sale securities, net of tax	161				161
Net loss	(18,451)			(18,451)
Stockholders' equity ending balance (in shares) at Jun. 30, 2020		54,166,712
Stockholders' equity ending balance at Jun. 30, 2020	$ 421,619	$ 54	1,202,445	(780,797)	(83)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in dollars per share)	$ 0.001
Stock-based compensation	$ 7,986		7,986
Issuance of common stock, net of expense (in shares)		5,594,593
Issuance of common stock, net of expense	492,414	$ 6	492,408
Exercise of stock options (in shares)		229,763
Exercise of stock options	8,284		8,284
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		10,879
Vesting of restricted stock units, net of shares withheld for taxes	(313)		(313)
Issuance of common stock related to ESPP (in shares)		13,026
Issuance of common stock related to ESPP	804		804
Net exercise of warrants to purchase common stock (in shares)		37,073
Net exercise of warrants to purchase common stock	3,729		3,729
Unrealized loss on available-for-sale securities, net of tax	(142)				(142)
Net loss	(39,245)			(39,245)
Stockholders' equity ending balance (in shares) at Sep. 30, 2020		60,052,046
Stockholders' equity ending balance at Sep. 30, 2020	$ 895,136	$ 60	1,715,343	(820,042)	(225)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in dollars per share)	$ 0.001
Common stock, par value (in dollars per share)	$ 0.001
Stockholders' equity beginning balance (in shares) at Dec. 31, 2020		60,383,867
Stockholders' equity beginning balance at Dec. 31, 2020	$ 855,107	$ 60	1,732,772	(877,437)	(288)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	15,626		15,626
Exercise of stock options (in shares)		149,724
Exercise of stock options	5,865		5,865
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		84,902
Vesting of restricted stock units, net of shares withheld for taxes	(3,888)		(3,888)
Issuance of common stock related to ESPP (in shares)		20,001
Issuance of common stock related to ESPP	1,721		1,721
Unrealized loss on available-for-sale securities, net of tax	(129)				(129)
Net loss	(63,471)			(63,471)
Stockholders' equity ending balance (in shares) at Mar. 31, 2021		60,638,494
Stockholders' equity ending balance at Mar. 31, 2021	810,831	$ 60	1,752,096	(940,908)	(417)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2020		60,383,867
Stockholders' equity beginning balance at Dec. 31, 2020	$ 855,107	$ 60	1,732,772	(877,437)	(288)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)	576,000
Unrealized loss on available-for-sale securities, net of tax	$ (110)
Net loss	(197,457)
Stockholders' equity ending balance (in shares) at Sep. 30, 2021		61,130,687
Stockholders' equity ending balance at Sep. 30, 2021	$ 718,153	$ 61	1,793,384	(1,074,894)	(398)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in dollars per share)	$ 0.001
Stockholders' equity beginning balance (in shares) at Mar. 31, 2021		60,638,494
Stockholders' equity beginning balance at Mar. 31, 2021	$ 810,831	$ 60	1,752,096	(940,908)	(417)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	13,479		13,479
Exercise of stock options (in shares)		100,450
Exercise of stock options	4,055		4,055
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		38,614
Vesting of restricted stock units, net of shares withheld for taxes	(1,818)	$ 1	(1,819)
Unrealized loss on available-for-sale securities, net of tax	8				8
Net loss	(63,521)			(63,521)
Stockholders' equity ending balance (in shares) at Jun. 30, 2021		60,777,558
Stockholders' equity ending balance at Jun. 30, 2021	$ 763,034	$ 61	1,767,811	(1,004,429)	(409)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in dollars per share)	$ 0.001
Stock-based compensation	$ 11,421		11,421
Exercise of stock options (in shares)		331,858
Exercise of stock options	13,822		13,822
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		12,335
Vesting of restricted stock units, net of shares withheld for taxes	(689)		(689)
Issuance of common stock related to ESPP (in shares)		8,936
Issuance of common stock related to ESPP	1,019		1,019
Unrealized loss on available-for-sale securities, net of tax	11				11
Net loss	(70,465)			(70,465)
Stockholders' equity ending balance (in shares) at Sep. 30, 2021		61,130,687
Stockholders' equity ending balance at Sep. 30, 2021	$ 718,153	$ 61	$ 1,793,384	$ (1,074,894)	$ (398)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock, par value (in dollars per share)	$ 0.001

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	3 Months Ended
	Sep. 30, 2020USD ($)$ / shares
Statement of Stockholders' Equity [Abstract]
Common stock, par value (in dollars per share) | $ / shares	$ 0.001
Issuance costs | $	$ 505

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Operating Activities
Net loss	$ (197,457)	$ (108,635)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,222	2,841
Stock-based compensation	40,526	21,805
Other non-cash items	(2,328)	1,552
Changes in assets and liabilities:
Prepaid expenses and other assets	(54,598)	1,231
Collaboration receivables (all amounts are with a related party)	(8,094)	(14,014)
Non-cash lease expense	4,849	4,249
Accounts payable	(3,418)	6,315
Accrued expenses	(795)	(3,025)
Operating lease obligations	(5,134)	(4,533)
Other changes in operating assets and liabilities	0	17
Net cash used in operating activities	(223,227)	(92,197)
Investing Activities
Purchases of investments	(407,580)	(161,741)
Proceeds from sales and maturities of investments	280,322	220,240
Purchases of property and equipment	(4,298)	(3,303)
Net cash (used in) provided by investing activities	(131,556)	55,196
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs	0	492,414
Net proceeds from exercises and vesting of stock awards, and ESPP contributions	20,086	36,595
Net cash provided by financing activities	20,086	529,009
Net (decrease) increase in cash, cash equivalents and restricted cash	(334,697)	492,008
Cash, cash equivalents and restricted cash at beginning of period	672,549	239,274
Cash, cash equivalents and restricted cash at end of period	337,852	731,282
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses	233	281
Additions to property and equipment for asset retirement obligation	373	0
Reclassification of warrant liability to additional paid-in capital	$ 0	$ 3,729

Nature of Business

Nature of Business	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business	Nature of Business Acceleron Pharma Inc. (Acceleron or the Company) is a Cambridge, Massachusetts-based biopharmaceutical company dedicated to the discovery, development and commercialization of therapeutics to treat serious and rare diseases. The Company’s leadership in the understanding of TGF-beta biology and protein engineering generates innovative compounds that engage the body’s ability to regulate cellular growth and repair. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, the risk that the Company never achieves profitability or successfully commercializes its products, the need for substantial additional financing, risks of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology, and compliance with government regulations. On September 29, 2021, Acceleron entered into an Agreement and Plan of Merger (the Merger Agreement) with Merck Sharp & Dohme Corp., a New Jersey corporation (Merck), and Merck’s wholly owned subsidiary, Astros Merger Sub, Inc., a Delaware corporation (Purchaser). Pursuant to the Merger Agreement, and upon the terms and subject to the conditions thereof, Purchaser has commenced a tender offer (the Offer) to purchase all of the issued and outstanding shares of common stock, par value $0.001 per share, of the Company in exchange for $180.00 per share, net to the seller in cash, without interest and less applicable tax withholdings. The closing of the Offer is subject to certain conditions, including the tender of shares representing at least a majority of the total number of Acceleron’s outstanding shares, receipt of applicable regulatory approvals, and other customary conditions. If these conditions are satisfied and the Offer closes, Merck would acquire any remaining shares by a merger of Purchaser with and into the Company. Acceleron expects the transaction to close in the fourth quarter of 2021. If the transaction is completed, it is expected that Acceleron's common stock will be removed from listing on the Nasdaq Global Market and from registration under Section 12(b) of the Securities Exchange Act of 1934, as amended.

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The accompanying interim condensed consolidated financial statements are unaudited and reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of September 30, 2021, the Company’s significant accounting policies and estimates, which are detailed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, have not changed, and the unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2020. In the opinion of management, the accompanying interim condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2021, the results of its operations for the three and nine months ended September 30, 2021 and 2020, stockholders' equity for the three and nine months ended September 30, 2021 and 2020, and its cash flows for the nine months ended September 30, 2021 and 2020. The accompanying interim condensed consolidated financial statements include the results of operations of the Company and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. The results for the three and nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, together with Management’s Discussion and Analysis of Financial Condition and Results of Operations, contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020.

Use of Estimates

Use of Estimates	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts expensed during the reporting period. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these condensed consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the condensed consolidated financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these condensed consolidated financial statements, management used estimates in the following areas, among others: accrued and prepaid clinical expenses, contract manufacturing expense, stock-based compensation expense, revenue recognition, and the recoverability of the Company's net deferred tax assets and related valuation allowance.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Segment Information	Segment InformationOperating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief executive officer view the Company’s operations and manage its business as one operating segment, which is the discovery, development and commercialization of highly innovative therapeutics to treat serious and rare diseases.

Cash, Cash Equivalents and Shor

Cash, Cash Equivalents and Short-term and Long-term Investments	9 Months Ended
	Sep. 30, 2021
Cash and Cash Equivalents [Abstract]
Cash, Cash Equivalents and Short-term and Long-term Investments	Cash, Cash Equivalents and Short-term and Long-term Investments The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in interest-bearing money market accounts. Cash equivalents are carried at cost, which approximates their fair value. The Company determines the appropriate classification of marketable securities at the time of purchase and reevaluates such designation at each balance sheet date. The Company has classified all of its marketable securities at September 30, 2021 as “available-for-sale” pursuant to ASC 320, Investments – Debt and Equity Securities. The Company records available-for-sale securities at fair value, with the unrealized gains and losses included in accumulated other comprehensive income (loss) in stockholders’ equity. There were no realized gains or losses on marketable securities for the nine months ended September 30, 2021 and 2020. Investments not classified as cash equivalents are presented as either short-term or long-term investments based on both their maturities as well as the time period the Company intends to hold such securities. The Company adjusts the cost of available-for-sale debt securities for amortization of premiums and accretion of discounts to maturity. The Company includes such amortization and accretion in interest income. The cost of securities sold is based on the specific identification method. The Company includes interest and dividends on securities classified as available-for-sale in interest income in the accompanying condensed consolidated statements of operations and comprehensive loss. Accrued interest receivable relating to the Company's available-for-sale securities is presented within prepaid expenses and other current assets in the accompanying condensed consolidated balance sheets, and amounted to $1.5 million and $0.3 million at September 30, 2021 and December 31, 2020, respectively. In June 2016, the FASB issued Accounting Standards Update 2016-13, Financial Instruments-Credit Losses . The new standard requires an estimate of expected credit losses only when the fair value of an available-for-sale debt security is below its amortized cost basis, and credit losses are limited to the amount by which the security’s amortized cost basis exceeds its fair value. Credit-related impairment is recognized as an allowance for credit losses on the balance sheet with a corresponding adjustment to earnings. The standard additionally requires an investor to determine whether a decline in the fair value below the amortized cost basis of an available-for-sale debt security is due to credit-related factors or noncredit-related factors. Any impairment that is not credit related is recognized in other comprehensive income, net of applicable taxes. The Company adopted ASU 2016-13 effective January 1, 2020, with no material impact on its consolidated financial statements and related disclosures. The following is a summary of available-for-sale securities with unrealized losses for less than 12 months as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 December 31, 2020 Fair Value Unrealized Losses Fair Value Unrealized Losses Corporate obligations $ 209,012 $ (98) $ 54,724 $ (13) U.S. Treasury securities 12,008 (1) 72,289 (7) Certificates of deposit 1,470 (1) — — Mortgage and other asset backed securities — — 4,998 (2) Total available-for sale securities in an unrealized loss position $ 222,490 $ (100) $ 132,012 $ (22) At September 30, 2021, our security portfolio consisted of 114 securities related to investments in debt securities available-for-sale, of which 79 securities were in an unrealized loss position. There were no securities in an unrealized loss position for greater than 12 months as of September 30, 2021. The unrealized losses on the Company's available-for-sale securities were caused by central bank and market interest rate decreases on securities purchased at a premium. The contractual terms of these investments do not permit the issuer to settle the securities at a price less than the amortized cost bases of the investments. The Company does not intend to sell the investments and it is not more likely than not that the Company will be required to sell the investments before recovery of their amortized cost bases. The Company did not record an allowance for credit losses as of September 30, 2021. The following is a summary of cash, cash equivalents and available-for-sale securities as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 336,456 $ — $ — $ 336,456 Available-for-sale securities: Corporate obligations 289,875 16 (98) 289,793 U.S. Treasury securities 24,554 2 (1) 24,555 Certificates of deposit 1,716 — (1) 1,715 Total available-for-sale securities (1) $ 316,145 $ 18 $ (100) $ 316,063 Total cash, cash equivalents and available-for-sale securities $ 652,601 $ 18 $ (100) $ 652,519 December 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 670,952 $ — $ — $ 670,952 Available-for-sale securities: Corporate obligations 45,989 5 (13) 45,981 U.S. Treasury securities 135,315 3 (7) 135,311 Certificates of deposit 245 1 — 246 Mortgage and other asset backed securities 5,000 — (2) 4,998 Total available-for-sale securities (1) $ 186,549 $ 9 $ (22) $ 186,536 Total cash, cash equivalents and available-for-sale securities $ 857,501 $ 9 $ (22) $ 857,488 (1) All available-for-sale securities mature within two and three years as of September 30, 2021 and December 31, 2020, respectively.

Restricted Cash

Restricted Cash	9 Months Ended
	Sep. 30, 2021
Restricted Cash [Abstract]
Restricted Cash	Restricted Cash The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same such amounts presented in the statements of cash flows (in thousands): September 30, 2021 2020 Cash and cash equivalents $ 336,456 $ 729,685 Restricted cash 1,396 1,597 Total cash, cash equivalents and restricted cash presented in the statement of cash flows $ 337,852 $ 731,282 As of September 30, 2021 and December 31, 2020, the Company maintained letters of credit totaling $1.4 million and $1.6 million, respectively, held in the form of certificates of deposit and money market funds as collateral for the Company's facility lease obligation and its credit cards. Restricted cash is included within other assets in the condensed consolidated balance sheet.

Concentrations of Credit Risk a

Concentrations of Credit Risk and Off-Balance Sheet Risk	9 Months Ended
	Sep. 30, 2021
Risks and Uncertainties [Abstract]
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off-Balance Sheet Risk The Company has no off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially subject the Company to concentrations of credit risk are primarily cash, cash equivalents, restricted cash, short-term and long-term investments and collaboration receivables. The Company maintains its cash and cash equivalent balances and short-term and long-term investments with financial institutions that management believes are creditworthy. Short-term and long-term investments consist of investment grade corporate obligations, treasury notes, asset backed securities, and certificates of deposit. The Company’s investment policy includes guidelines on the quality of the institutions and financial instruments and defines allowable investments that the Company believes minimizes the exposure to concentrations of credit risk. The Company routinely assesses the creditworthiness of its collaboration partner. The Company has not experienced any material losses related to receivables from individual customers and collaboration partners, or groups of customers. The Company does not require collateral. Due to these factors, no allowance for credit losses has been recorded for the Company's collaboration receivables as of September 30, 2021 and December 31, 2020.

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 set forth the Company’s financial instruments carried at fair value using the lowest level of input that is significant to each financial instrument as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 259,772 $ — $ — $ 259,772 Corporate obligations — 292,835 — 292,835 U.S. Treasury securities — 35,556 — 35,556 Certificates of deposit — 8,994 — 8,994 Total assets $ 259,772 $ 337,385 $ — $ 597,157 December 31, 2020 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 266,562 $ — $ — $ 266,562 Corporate obligations — 60,982 — 60,982 U.S. Treasury securities — 215,310 — 215,310 Certificates of deposit — 246 — 246 Mortgage and other asset backed securities — 4,998 — 4,998 Total assets $ 266,562 $ 281,536 $ — $ 548,098 The money market funds noted above are included in cash and cash equivalents in the accompanying condensed consolidated balance sheets. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy during the nine months ended September 30, 2021 or the year ended December 31, 2020. Items measured at fair value on a recurring basis include short-term and long-term investments (Note 5).

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 ShareThe following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because their inclusion would have had an anti-dilutive effect (in thousands): Three and Nine Months Ended September 30, 2021 2020 Outstanding stock options $ 3,447 $ 3,546 Common stock warrants — — Shares issuable under employee stock purchase plan 20 19 Outstanding restricted stock units (1) 693 572 $ 4,160 $ 4,137 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 15.

Comprehensive Loss

Comprehensive Loss	9 Months Ended
	Sep. 30, 2021
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Comprehensive Loss	Comprehensive LossComprehensive loss is defined as the change in equity of a business enterprise during a period from transactions, other events, and circumstances from non-owner sources. Comprehensive loss consists of net loss and other comprehensive income (loss), which includes certain changes in equity that are excluded from net loss. Comprehensive loss has been disclosed in the accompanying condensed consolidated statements of operations and comprehensive loss. Accumulated other comprehensive income (loss) is presented separately on the condensed consolidated balance sheets and consists entirely of unrealized holdings gains or losses on investments as of September 30, 2021 and December 31, 2020.

Prepaid Expense and Other Curre

Prepaid Expense and Other Current Assets	9 Months Ended
	Sep. 30, 2021
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expense and Other Current Assets	Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following (in thousands): September 30, 2021 December 31, 2020 Prepaid clinical trial expenses $ 61,816 $ 7,930 Accrued interest 1,470 345 Other prepaid and current assets 8,558 8,990 Total prepaid expenses and other current assets $ 71,844 $ 17,265 In 2021, prepaid clinical trial expenses increased primarily due to upfront payments made to fund the sotatercept clinical trials.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Operating Leases The Company leases its facilities under non-cancelable operating leases that expire at various dates from September 2023 through May 2026. All of the Company's leases contain escalating rent clauses, which require higher rent payments in future years. See Note 2 and Note 14 in our Annual Report on Form 10-K for the year ended December 31, 2020 for additional information regarding the Company's operating leases. Legal Proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of its business. The Company was not subject to any material legal proceedings during the three and nine months ended September 30, 2021. For a description of the Company's legal proceedings since September 30, 2021, see "Part II. Other Information - Item 1. Legal Proceedings" of this Quarterly Report on Form 10-Q. Merger Agreement In connection with the termination of the Merger Agreement under specified circumstances, including termination by Acceleron in order to accept and enter into a definitive agreement with respect to a Superior Proposal (as defined in the Merger Agreement), we will be required to pay Merck a termination fee of $345.0 million by wire transfer of immediately available funds. The Merger Agreement also provides that if the tender offer or merger are not consummated due to the failure of certain antitrust conditions to be satisfied, then Merck will be required to pay us a reverse termination fee of $650.0 million or $750.0 million, depending on the time of termination. The termination fees, if paid, constitute the sole and exclusive remedy of the party receiving the fee in respect of the Merger Agreement. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at September 30, 2021 and December 31, 2020, or royalties on future sales of specified products. See Note 13 for discussion of these arrangements. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to the agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners or customers, in connection with any U.S. patent or any copyright or other intellectual property infringement claim by any third party with respect to the Company’s products. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements.

Significant Agreements

Significant Agreements	9 Months Ended
	Sep. 30, 2021
Significant Agreements [Abstract]
Significant Agreements	Significant Agreements BMS (Bristol Myers Squibb Company) Overview On February 20, 2008, the Company entered into an agreement with Celgene, which was acquired by BMS in November 2019 and is now referred to herein as BMS, relating to sotatercept (the Original Sotatercept Agreement), which was amended on August 2, 2011 (as amended, the Amended Sotatercept Agreement). The Company further amended and restated the Original Sotatercept Agreement in its entirety on September 18, 2017, and clarified certain responsibilities of the Company and BMS in a letter agreement to the Restated Sotatercept Agreement on March 10, 2020 (collectively, the Restated Sotatercept Agreement). On August 2, 2011, the Company entered into a second agreement with BMS for REBLOZYL® (luspatercept-aamt) (the REBLOZYL Agreement, formerly the Luspatercept Agreement). Since December 31, 2020, there have been no material changes to the key terms of the above agreements. For further information on the terms of the agreements, please see the notes to the consolidated financial statements included in the Company’s Form 10-K for the year ended December 31, 2020. Accounting Analysis Upon adoption of ASC 606, all of the Company’s performance obligations pursuant to its arrangements with BMS were completed and all remaining potential milestone payments were fully constrained as they related to future regulatory events that were outside of the Company’s control, and therefore the risk of significant reversal in the amount of cumulative revenue had not been resolved. As of September 30, 2021, the last remaining regulatory milestone payment for REBLOZYL would be $20.0 million and would result from approval by a regulatory authority in Asia (as defined in the REBLOZYL Agreement) of a Biologics License Application (BLA) or equivalent for luspatercept-aamt in either myelodysplastic syndromes or beta-thalassemia. In accordance with the Company's accounting policy regarding revenue recognition as described in Note 2 to its Annual Report on Form 10-K, the revenue associated with this milestone will be recognized once it is probable that the application is approved by the regulatory authority. Milestone payments that are not within the control of the Company or the licensee are not considered probable of being achieved until those approvals are received. The approval of the application is not within the control of the Company or the licensee, and therefore, as of September 30, 2021, the Company cannot determine if it is probable that a regulatory agency will approve the applications. During the three and nine months ended September 30, 2021 and 2020, the Company recognized milestone, cost-sharing, and royalty revenue pursuant to the BMS arrangements, as presented on the condensed consolidated statements of operations and comprehensive loss. Through September 30, 2021, under all BMS arrangements, the Company has received net cost-share payments, milestones, and royalties of $335.3 million and $45.1 million for REBLOZYL and sotatercept, respectively. Other Agreements In 2004, the Company entered into a license agreement with a non-profit institution for an exclusive, sublicensable, worldwide, royalty-bearing license to certain patents developed by the institution (Primary Licensed Products). In addition, the Company was granted a non-exclusive, non-sub-licensable license for Secondary Licensed Products. The Company agreed to pay specified development milestone payments totaling up to $2.0 million for sotatercept and $0.7 million for REBLOZYL. In addition, the Company is obligated to pay milestone fees based on the Company’s research and development progress, and U.S. sublicensing revenue ranging from 10%-25%, as well as royalties ranging from 1.0%-3.5% of net sales on any products developed under the licenses. During the three months ended September 30, 2021 and 2020, the Company did not recognize any expense related milestones and fees. During the nine months ended September 30, 2021 and 2020, the Company expensed zero and $1.8 million, respectively, of milestones and fees. Milestones and fees associated with development related activities are recorded as research and development expense. During the three months ended September 30, 2021 and 2020, the Company expensed $1.6 million and $1.0 million, respectively, and during the nine months ended September 30, 2021 and 2020, the Company expensed $4.1 million and $1.6 million, respectively, of royalties. Costs related to royalties on sales of commercial products are recorded as selling, general and administrative expense. In May 2014, the Company executed a collaboration agreement with a research technology company, and such collaboration agreement was amended and restated in March 2019. The Company paid an upfront research fee of $0.3 million upon execution of the original agreement. The Company also received an option to obtain a commercial license to the molecules developed during the collaboration. During the three months ended September 30, 2021 and 2020, the Company expensed $0.6 million and $0.1 million, respectively, and during the nine months ended September 30, 2021 and 2020, the Company expensed $0.6 million of milestones and fees, which is recorded as research and development expense. In December 2019, the Company executed a license and collaboration agreement with Fulcrum Therapeutics to identify small molecules designed to modulate specific pathways associated with a targeted indication within the pulmonary disease space. The Company paid an upfront research fee of $10.0 million upon execution of this agreement, which was expensed to research and development. The Company also agreed to pay specified research, development and commercial milestone payments of up to $295.0 million for a first product commercialized and up to a maximum of $143.5 million in additional milestone payments for all subsequent products commercialized. Fulcrum will additionally receive tiered royalty payments in the mid-single-digit to low double-digit range on net sales, as well as reimbursement for relevant research and development costs. During the three months ended September 30, 2021 and 2020, the Company expensed $0.6 million and during the nine months ended September 30, 2021 and 2020, the Company expensed $1.7 million of milestones and fees, which is recorded as research and development expense.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Stockholders' Equity	Stockholders' EquityOn July 6, 2020, the Company completed the sale in an underwritten public offering of 5,594,593 shares of common stock, including 729,729 shares of common stock sold pursuant to the underwriter's full exercise of their option to purchase additional shares, at a public offering price of $92.50 per share, resulting in net proceeds to the Company of $492.4 million.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation During the quarter ended March 31, 2021, the Company updated the equity award agreements with respect to the 2013 Equity Incentive Plan (the 2013 Plan) to enhance the vesting terms for retirement eligible employees and directors for awards granted in 2021 and future awards. Where awards are granted with non-substantive vesting periods (for instance, where all or a portion of the award continues to vest upon retirement eligibility), the Company recognizes expense based on the period from the grant date to the date the employee becomes retirement eligible. The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Plan, and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the condensed consolidated statements of operations and comprehensive loss during the three and nine months ended September 30, 2021 and 2020, respectively, as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Research and development $ 5,586 $ 3,794 $ 19,615 $ 10,194 Selling, general and administrative 5,835 4,192 20,911 11,611 $ 11,421 $ 7,986 $ 40,526 $ 21,805 Stock Options The following table summarizes the stock option activity under the Company’s stock option plans during the nine months ended September 30, 2021 (in thousands, except per share amounts and years): Number Weighted- Weighted- Aggregate Outstanding at December 31, 2020 3,378 $ 49.78 7.35 $ 264,046 Granted 769 $ 119.78 Exercised (576) $ 41.18 Canceled or forfeited (123) $ 80.33 Outstanding at September 30, 2021 3,448 $ 65.74 7.41 $ 366,668 Exercisable at September 30, 2021 1,722 $ 45.23 6.18 $ 218,481 (1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2021. The aggregate intrinsic value of options exercised during the nine months ended September 30, 2021 was $55.4 million. As of September 30, 2021, there was approximately $66.7 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a remaining weighted-average period of 2.50 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the nine months ended September 30, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 494 $ 59.16 Granted 269 $ 119.54 Vested (187) $ 47.69 Forfeited (34) $ 80.46 Unvested balance at September 30, 2021 542 $ 91.72 As of September 30, 2021, there was approximately $36.5 million of unrecognized compensation expense related to RSUs that is expected to be recognized over a remaining weighted-average period of 1.79 years. Performance-Based Restricted Stock Units On January 22, 2020, the Company granted performance-based restricted stock units (PSU) whereby vesting depends upon the occurrence of certain milestone events by December 31, 2022 (the 2020 PSUs). When achievement of a milestone event becomes probable, compensation cost will be recognized from the grant date over the requisite service period and a cumulative catch-up adjustment will be recorded to reflect the portion of the employees' requisite service that has been provided to date. During the nine months ended September 30, 2021, stock-based compensation expense of $2.2 million related to the 2020 PSUs was recognized for one of the milestone events deemed probable of achievement. The expense associated with these PSUs will continue to be recognized through December 31, 2022. For the remaining two milestone events which were not deemed probable of achievement, no stock-based compensation expense was recognized. On January 29, 2021, the Company granted performance-based restricted stock units (PSU) whereby vesting depends upon the occurrence of certain milestone events by December 31, 2023 (the 2021 PSUs). When achievement of a milestone event becomes probable, compensation cost will be recognized from the grant date over the requisite service period and a cumulative catch-up adjustment will be recorded to reflect the portion of the employees' requisite service that has been provided to date. As of September 30, 2021, none of the milestone events related to the 2021 PSUs had been deemed probable of being achieved. The following table summarizes PSU activity under the 2013 Plan during the nine months ended September 30, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 75 $ 52.99 Granted (1) 83 $ 115.53 Vested — $ — Forfeited (7) $ 76.10 Unvested balance at September 30, 2021 151 $ 86.04 (1) Pursuant to the terms of the awards granted, the actual number of awards earned could range between 0% and 200% of the number of awards granted. As of September 30, 2021, there was approximately $10.8 million of related unrecognized compensation cost. Depending on the actual number of milestone events achieved, the actual expense recognized could range between 0% and 200% of the total target value awarded on the grant date.

Income Taxes

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

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions BMS BMS owned 10.7% and 10.8% of the Company’s fully diluted equity as of September 30, 2021 and December 31, 2020, respectively. Refer to Note 13 for additional information regarding this collaboration arrangement.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	Subsequent EventsThe Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The accompanying interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The accompanying interim condensed consolidated financial statements are unaudited and reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the financial statements. As of September 30, 2021, the Company’s significant accounting policies and estimates, which are detailed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, have not changed, and the unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2020. In the opinion of management, the accompanying interim condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2021, the results of its operations for the three and nine months ended September 30, 2021 and 2020, stockholders' equity for the three and nine months ended September 30, 2021 and 2020, and its cash flows for the nine months ended September 30, 2021 and 2020. The accompanying interim condensed consolidated financial statements include the results of operations of the Company and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. The results for the three and nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the year ending December 31, 2021, any other interim periods, or any future year or period. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2020, and the notes thereto, together with Management’s Discussion and Analysis of Financial Condition and Results of Operations, contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020.
Use of Estimates	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts expensed during the reporting period. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these condensed consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the condensed consolidated financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these condensed consolidated financial statements, management used estimates in the following areas, among others: accrued and prepaid clinical expenses, contract manufacturing expense, stock-based compensation expense, revenue recognition, and the recoverability of the Company's net deferred tax assets and related valuation allowance.

Cash, Cash Equivalents and Sh_2

Cash, Cash Equivalents and Short-term and Long-term Investments (Tables)	9 Months Ended
	Sep. 30, 2021
Cash and Cash Equivalents [Abstract]
Available-for-Sale Securities with Unrealized Losses	The following is a summary of available-for-sale securities with unrealized losses for less than 12 months as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 December 31, 2020 Fair Value Unrealized Losses Fair Value Unrealized Losses Corporate obligations $ 209,012 $ (98) $ 54,724 $ (13) U.S. Treasury securities 12,008 (1) 72,289 (7) Certificates of deposit 1,470 (1) — — Mortgage and other asset backed securities — — 4,998 (2) Total available-for sale securities in an unrealized loss position $ 222,490 $ (100) $ 132,012 $ (22)
Available-for-Sale Securities	The following is a summary of cash, cash equivalents and available-for-sale securities as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 336,456 $ — $ — $ 336,456 Available-for-sale securities: Corporate obligations 289,875 16 (98) 289,793 U.S. Treasury securities 24,554 2 (1) 24,555 Certificates of deposit 1,716 — (1) 1,715 Total available-for-sale securities (1) $ 316,145 $ 18 $ (100) $ 316,063 Total cash, cash equivalents and available-for-sale securities $ 652,601 $ 18 $ (100) $ 652,519 December 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 670,952 $ — $ — $ 670,952 Available-for-sale securities: Corporate obligations 45,989 5 (13) 45,981 U.S. Treasury securities 135,315 3 (7) 135,311 Certificates of deposit 245 1 — 246 Mortgage and other asset backed securities 5,000 — (2) 4,998 Total available-for-sale securities (1) $ 186,549 $ 9 $ (22) $ 186,536 Total cash, cash equivalents and available-for-sale securities $ 857,501 $ 9 $ (22) $ 857,488 (1) All available-for-sale securities mature within two and three years as of September 30, 2021 and December 31, 2020, respectively.

Restricted Cash (Tables)

Restricted Cash (Tables)	9 Months Ended
	Sep. 30, 2021
Restricted Cash [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same such amounts presented in the statements of cash flows (in thousands): September 30, 2021 2020 Cash and cash equivalents $ 336,456 $ 729,685 Restricted cash 1,396 1,597 Total cash, cash equivalents and restricted cash presented in the statement of cash flows $ 337,852 $ 731,282
Restrictions on Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same such amounts presented in the statements of cash flows (in thousands): September 30, 2021 2020 Cash and cash equivalents $ 336,456 $ 729,685 Restricted cash 1,396 1,597 Total cash, cash equivalents and restricted cash presented in the statement of cash flows $ 337,852 $ 731,282

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Schedule of Financial Instruments Carried at Fair Value	The following tables set forth the Company’s financial instruments carried at fair value using the lowest level of input that is significant to each financial instrument as of September 30, 2021 and December 31, 2020 (in thousands): September 30, 2021 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 259,772 $ — $ — $ 259,772 Corporate obligations — 292,835 — 292,835 U.S. Treasury securities — 35,556 — 35,556 Certificates of deposit — 8,994 — 8,994 Total assets $ 259,772 $ 337,385 $ — $ 597,157 December 31, 2020 Quoted Prices Significant Other Significant Total Assets: Money market funds $ 266,562 $ — $ — $ 266,562 Corporate obligations — 60,982 — 60,982 U.S. Treasury securities — 215,310 — 215,310 Certificates of deposit — 246 — 246 Mortgage and other asset backed securities — 4,998 — 4,998 Total assets $ 266,562 $ 281,536 $ — $ 548,098

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Common Stock Equivalents Excluded from the Calculation of Diluted Net Loss Per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because their inclusion would have had an anti-dilutive effect (in thousands): Three and Nine Months Ended September 30, 2021 2020 Outstanding stock options $ 3,447 $ 3,546 Common stock warrants — — Shares issuable under employee stock purchase plan 20 19 Outstanding restricted stock units (1) 693 572 $ 4,160 $ 4,137 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 15.

Prepaid Expense and Other Cur_2

Prepaid Expense and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following (in thousands): September 30, 2021 December 31, 2020 Prepaid clinical trial expenses $ 61,816 $ 7,930 Accrued interest 1,470 345 Other prepaid and current assets 8,558 8,990 Total prepaid expenses and other current assets $ 71,844 $ 17,265

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Schedule of Compensation Costs Recognized	The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Plan, and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the condensed consolidated statements of operations and comprehensive loss during the three and nine months ended September 30, 2021 and 2020, respectively, as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Research and development $ 5,586 $ 3,794 $ 19,615 $ 10,194 Selling, general and administrative 5,835 4,192 20,911 11,611 $ 11,421 $ 7,986 $ 40,526 $ 21,805
Summary of Stock Option Activity	The following table summarizes the stock option activity under the Company’s stock option plans during the nine months ended September 30, 2021 (in thousands, except per share amounts and years): Number Weighted- Weighted- Aggregate Outstanding at December 31, 2020 3,378 $ 49.78 7.35 $ 264,046 Granted 769 $ 119.78 Exercised (576) $ 41.18 Canceled or forfeited (123) $ 80.33 Outstanding at September 30, 2021 3,448 $ 65.74 7.41 $ 366,668 Exercisable at September 30, 2021 1,722 $ 45.23 6.18 $ 218,481 (1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2021.
Schedule of Nonvested Restricted Stock Units Activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the nine months ended September 30, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 494 $ 59.16 Granted 269 $ 119.54 Vested (187) $ 47.69 Forfeited (34) $ 80.46 Unvested balance at September 30, 2021 542 $ 91.72
Share-Based Compensation, Performance Shares Award Nonvested Activity	The following table summarizes PSU activity under the 2013 Plan during the nine months ended September 30, 2021 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2020 75 $ 52.99 Granted (1) 83 $ 115.53 Vested — $ — Forfeited (7) $ 76.10 Unvested balance at September 30, 2021 151 $ 86.04 (1) Pursuant to the terms of the awards granted, the actual number of awards earned could range between 0% and 200% of the number of awards granted.

Nature of Business (Details)

Nature of Business (Details) - $ / shares	Sep. 30, 2021	Sep. 29, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001
Price per share (in dollars per share)		$ 180

Segment Information (Details)

Segment Information (Details)	9 Months Ended
	Sep. 30, 2021segment
Segment Information
Number of operating segments	1

Cash, Cash Equivalents and Sh_3

Cash, Cash Equivalents and Short-term and Long-term Investments - Narrative (Details) $ in Thousands	Sep. 30, 2021USD ($)debt_security	Dec. 31, 2020USD ($)
Cash and Cash Equivalents [Abstract]
Accrued interest | $	$ 1,470	$ 345
Number of positions	114
Unrealized loss position, number of positions	79
Continuous unrealized loss position, 12 months or longer, number of positions	0

Cash, Cash Equivalents and Sh_4

Cash, Cash Equivalents and Short-term and Long-term Investments - Summary of Available-for-sale Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 222,490	$ 132,012
Unrealized Losses	(100)	(22)
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	209,012	54,724
Unrealized Losses	(98)	(13)
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	12,008	72,289
Unrealized Losses	(1)	(7)
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Fair Value	1,470	0
Unrealized Losses	(1)	0
Mortgage and other asset backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	4,998
Unrealized Losses	$ 0	$ (2)

Cash, Cash Equivalents and Sh_5

Cash, Cash Equivalents and Short-term and Long-term Investments - Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Cash and cash equivalents due in 90 days or less
Amortized Cost	$ 336,456	$ 670,952
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	336,456	670,952	$ 729,685
Available-for-sale securities:
Amortized Cost	316,145	186,549
Gross Unrealized Gains	18	9
Gross Unrealized Losses	(100)	(22)
Estimated Fair Value	316,063	186,536
Total cash, cash equivalents and available-for-sale securities
Amortized Cost	652,601	857,501
Gross Unrealized Gains	18	9
Gross Unrealized Losses	(100)	(22)
Estimated Fair Value	652,519	857,488
Corporate obligations
Available-for-sale securities:
Amortized Cost	289,875	45,989
Gross Unrealized Gains	16	5
Gross Unrealized Losses	(98)	(13)
Estimated Fair Value	289,793	45,981
U.S. Treasury securities
Available-for-sale securities:
Amortized Cost	24,554	135,315
Gross Unrealized Gains	2	3
Gross Unrealized Losses	(1)	(7)
Estimated Fair Value	24,555	135,311
Certificates of deposit
Available-for-sale securities:
Amortized Cost	1,716	245
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	0
Estimated Fair Value	$ 1,715	246
Mortgage and other asset backed securities
Available-for-sale securities:
Amortized Cost		5,000
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Estimated Fair Value		$ 4,998

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 336,456	$ 670,952	$ 729,685
Restricted cash	1,396		1,597
Total cash, cash equivalents and restricted cash presented in the statement of cash flows	337,852	672,549	$ 731,282	$ 239,274
Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 1,400	$ 1,600

Concentrations of Credit Risk_2

Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Risks and Uncertainties [Abstract]
Allowance for credit losses	$ 0	$ 0

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Available-for-sale debt securities	$ 316,063	$ 186,536
Corporate obligations
Assets:
Available-for-sale debt securities	289,793	45,981
Certificates of deposit
Assets:
Available-for-sale debt securities	1,715	246
Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		4,998
Recurring
Assets:
Total assets	597,157	548,098
Recurring | Money market funds
Assets:
Available-for-sale debt securities	259,772	266,562
Recurring | Corporate obligations
Assets:
Available-for-sale debt securities	292,835	60,982
Recurring | U.S. Treasury securities
Assets:
Available-for-sale debt securities	35,556	215,310
Recurring | Certificates of deposit
Assets:
Available-for-sale debt securities	8,994	246
Recurring | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		4,998
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	259,772	266,562
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Available-for-sale debt securities	259,772	266,562
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	337,385	281,536
Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Corporate obligations
Assets:
Available-for-sale debt securities	292,835	60,982
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	35,556	215,310
Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Available-for-sale debt securities	8,994	246
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		4,998
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Available-for-sale debt securities	$ 0	0
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities		$ 0

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,160	4,137	4,160	4,137
Outstanding stock options
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	3,447	3,546	3,447	3,546
Common stock warrants
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	0	0	0	0
Shares issuable under employee stock purchase plan
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	20	19	20	19
Outstanding restricted stock units
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	693	572	693	572

Prepaid Expense and Other Cur_3

Prepaid Expense and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid clinical trial expenses	$ 61,816	$ 7,930
Accrued interest	1,470	345
Other prepaid and current assets	8,558	8,990
Prepaid expenses and other current assets	$ 71,844	$ 17,265

Commitments and Contingencies (

Commitments and Contingencies (Details) - Merger Agreement Termination Fee $ in Millions	Sep. 30, 2021USD ($)
Loss Contingencies [Line Items]
Merck termination fees	$ 345
Merck | Minimum
Loss Contingencies [Line Items]
Merck termination fees	650
Merck | Maximum
Loss Contingencies [Line Items]
Merck termination fees	$ 750

Significant Agreements - Celgen

Significant Agreements - Celgene, Accounting Analysis (Details) - Collaboration, License, and Option Agreement $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
REBLOZYL Agreement
Significant Agreements
Payments received	$ 335.3
Sotarcept
Significant Agreements
Payments received	45.1
Regulatory Milestone | Luspatercept
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 20

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2019	May 31, 2014	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2004
Significant Agreements
Milestones and fees			$ 59,717,000	$ 40,747,000	$ 173,146,000	$ 116,663,000
Selling, general and administrative			45,112,000	21,042,000	111,646,000	59,705,000
License agreement with Antibody Technology Company | Additional Research Fees
Significant Agreements
Collaborative arrangement amount paid and expensed		$ 300,000
License Agreement With Fulcrum Therapeutics
Significant Agreements
Milestones and fees			600,000	600,000	1,700,000	1,700,000
License Agreement With Fulcrum Therapeutics | Additional Research Fees
Significant Agreements
Collaborative arrangement amount paid and expensed	$ 10,000,000
License Agreement With Fulcrum Therapeutics | Additional Research Fees | Maximum
Significant Agreements
Total potential milestone payments (up to)	143,500,000
License Agreement With Fulcrum Therapeutics | Research, Development and Commercial Milestone Payments | Maximum
Significant Agreements
Total potential milestone payments (up to)	$ 295,000,000
Non-collaborative Arrangement Transactions | Sotarcept | Development Milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)							$ 2,000,000
Non-collaborative Arrangement Transactions | REBLOZYL Agreement | Development Milestone | Maximum
Significant Agreements
Total potential milestone payments (up to)							$ 700,000
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution
Significant Agreements
Milestones and fees			0	0	0	1,800,000
Selling, general and administrative			1,600,000	1,000,000	4,100,000	1,600,000
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution | Minimum
Significant Agreements
Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue							10.00%
Royalty payable as percentage of net sales							1.00%
Non-collaborative Arrangement Transactions | License Agreement with Non Profit Institution | Maximum
Significant Agreements
Milestone fees payable as percentage of research and U.S. development progress and sublicensing revenue							25.00%
Royalty payable as percentage of net sales							3.50%
Non-collaborative Arrangement Transactions | License agreement with Antibody Technology Company
Significant Agreements
Milestones and fees			$ 600,000	$ 100,000	$ 600,000	$ 600,000

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	Jul. 06, 2020	Sep. 30, 2020	Sep. 29, 2021
Class of Stock [Line Items]
Price per share (in dollars per share)			$ 180
Common Stock
Class of Stock [Line Items]
Issuance of common stock, net of expense (in shares)	5,594,593	5,594,593
Public Stock Offering
Class of Stock [Line Items]
Price per share (in dollars per share)	$ 92.50
Net proceeds on transaction	$ 492.4
Over-Allotment Option
Class of Stock [Line Items]
Number of shares issued in transaction (in shares)	729,729

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 11,421	$ 7,986	$ 40,526	$ 21,805
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	5,586	3,794	19,615	10,194
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 5,835	$ 4,192	$ 20,911	$ 11,611

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Number of Shares
Outstanding at the beginning of period (in shares) | shares	3,378
Granted (in shares) | shares	769
Exercised (in shares) | shares	(576)
Canceled or forfeited (in shares) | shares	(123)
Outstanding at the end of the period (in shares) | shares	3,448	3,378
Exercisable at the end of the period (in shares) | shares	1,722
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in dollars per share) | $ / shares	$ 49.78
Granted (in dollars per share) | $ / shares	119.78
Exercised (in dollars per share) | $ / shares	41.18
Canceled or forfeited (in dollars per share) | $ / shares	80.33
Outstanding at the end of the period (in dollars per share) | $ / shares	65.74	$ 49.78
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 45.23
Weighted- Average Remaining Contractual Life  (in years)
Outstanding at the beginning of the period	7 years 4 months 28 days	7 years 4 months 6 days
Outstanding at the end of the period	7 years 4 months 28 days	7 years 4 months 6 days
Exercisable at the end of the period	6 years 2 months 4 days
Aggregate Intrinsic Value
Outstanding at period start | $	$ 264,046
Outstanding at period end | $	366,668	$ 264,046
Exercisable at the end of the period | $	218,481
Aggregate intrinsic value of options exercised | $	55,400
Unrecognized compensation expense | $	$ 66,700
Weighted-average period for recognition	2 years 6 months

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares
Weighted- Average Grant Date Fair Value
Weighted-average period for recognition	2 years 6 months
Employee benefits and share-based compensation | $	$ 2.2
Minimum
Weighted- Average Grant Date Fair Value
Actual awards earned percentage of expected expense recognized	0.00%
Maximum
Weighted- Average Grant Date Fair Value
Actual awards earned percentage of expected expense recognized	200.00%
Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	494
Granted (in shares) | shares	269
Vested (in shares) | shares	(187)
Forfeited (in shares) | shares	(34)
Unvested balance at end of the period (in shares) | shares	542
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in dollars per share) | $ / shares	$ 59.16
Granted (in dollars per share) | $ / shares	119.54
Vested (in dollars per share) | $ / shares	47.69
Forfeited (in dollars per share) | $ / shares	80.46
Unvested balance at end of the period (in dollars per share) | $ / shares	$ 91.72
Unrecognized compensation cost | $	$ 36.5
Weighted-average period for recognition	1 year 9 months 14 days
Performance-Based Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	75
Granted (in shares) | shares	83
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(7)
Unvested balance at end of the period (in shares) | shares	151
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in dollars per share) | $ / shares	$ 52.99
Granted (in dollars per share) | $ / shares	115.53
Vested (in dollars per share) | $ / shares	0
Forfeited (in dollars per share) | $ / shares	76.10
Unvested balance at end of the period (in dollars per share) | $ / shares	$ 86.04
Unrecognized compensation cost | $	$ 10.8

Related Party Transactions (Det

Related Party Transactions (Details)	Sep. 30, 2021	Dec. 31, 2020
Bristol-Myers Squibb Company (BMS, formerly Celgene) | Bristol-Myers Squibb Company (BMS, formerly Celgene)
Related Party Transactions
Ownership percentage of entity's fully diluted equity	10.70%	10.80%