Acceleron Pharma (XLRN) 10-Q 11 May 20 2020 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
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)		53,824,625
Entity Central Index Key	0001280600
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 277,486	$ 237,677
Collaboration receivables (all amounts are with a related party)	8,435	8,547
Prepaid expenses and other current assets	9,284	10,000
Short-term investments	136,954	193,692
Total current assets	432,159	449,916
Property and equipment, net	6,668	6,812
Operating lease - right of use asset, net	22,531	23,908
Other assets	1,681	1,793
Long-term investments	1,123	22,477
Total assets	464,162	504,906
Current liabilities:
Accounts payable	3,929	2,295
Accrued expenses	18,189	24,895
Operating lease liability - right of use	6,412	6,183
Total current liabilities	28,530	33,373
Operating lease liability - right of use, net of current portion	18,506	20,201
Warrants to purchase common stock	3,304	1,856
Total liabilities	50,340	55,430
Commitments and contingencies (Note 13)
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; 53,519,920 and 53,123,567 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	53	53
Additional paid-in capital	1,176,359	1,160,807
Accumulated deficit	(762,346)	(711,407)
Accumulated other comprehensive (loss) income	(244)	23
Total stockholders’ equity	413,822	449,476
Total liabilities and stockholders’ equity	$ 464,162	$ 504,906

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
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 usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	175,000,000	175,000,000
Common stock, shares issued (in shares)	53,519,920	53,123,567
Common stock, shares outstanding (in shares)	53,519,920	53,123,567

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Collaboration revenue:
Revenues	$ 4,344	$ 2,780
Costs and expenses:
Research and development	37,663	32,771
Selling, general and administrative	18,253	10,814
Total costs and expenses	55,916	43,585
Loss from operations	(51,572)	(40,805)
Other income, net	648	2,772
Loss before income taxes	(50,924)	(38,033)
Income tax provision	(15)	(20)
Net loss	(50,939)	(38,053)
Other comprehensive (loss) income:
Net unrealized holding (losses) gains on short-term and long-term investments during the period, net of tax	(267)	268
Comprehensive loss	$ (51,206)	$ (37,785)
Net loss per share- basic and diluted (in dollars per share)	$ (0.95)	$ (0.74)
Weighted-average number of common shares used in computing net loss per share- basic and diluted (in shares)	53,361	51,126
Cost-sharing, net
Collaboration revenue:
Revenues	$ 2,824	$ 2,780
Royalty
Collaboration revenue:
Revenues	$ 1,520	$ 0

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Comprehensive Income (Loss)
Stockholders' equity beginning balance (in shares) at Dec. 31, 2018		46,260,747
Stockholders' equity beginning balance at Dec. 31, 2018	$ 292,037	$ 47	$ 879,099	$ (586,549)	$ (560)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	6,992		6,992
Issuance of common stock, net of expense (in shares)		6,151,163
Issuance of common stock, net of expense	248,130	$ 6	248,124
Exercise of stock options (in shares)		35,919
Exercise of stock options	766		766
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		75,028
Vesting of restricted stock units, net of shares withheld for taxes	(393)		(393)
Issuance of common stock related to ESPP (in shares)		19,661
Issuance of common stock related to ESPP	788		788
Unrealized loss on available-for-sale securities, net of tax	268				268
Net loss	(38,053)			(38,053)
Stockholders' equity ending balance (in shares) at Mar. 31, 2019		52,542,518
Stockholders' equity ending balance at Mar. 31, 2019	510,535	$ 53	1,135,376	(624,602)	(292)
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)	296,000	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)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance costs	$ 500

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Operating Activities
Net loss		$ (50,939)	$ (38,053)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		996	933
Stock-based compensation		6,679	6,992
Other non-cash items		1,365	959
Changes in assets and liabilities:
Prepaid expenses and other assets		828	(237)
Collaboration receivables (all amounts are with a related party)		112	4,259
Non-cash lease expense	[1]	1,377	1,234
Accounts payable		1,634	3,466
Accrued expenses		(7,148)	(4,544)
Operating lease obligations		(1,466)	(1,317)
Other changes in operating assets and liabilities		17	0
Net cash used in operating activities		(46,545)	(26,308)
Investing Activities
Purchases of investments		(10,454)	(216,116)
Proceeds from sales and maturities of investments		88,361	64,133
Purchases of property and equipment		(426)	(587)
Net cash provided by (used in) investing activities		77,481	(152,570)
Financing Activities
Proceeds from issuance of common stock from public offering, net of issuance costs		0	248,155
Net proceeds from exercises and vesting of stock awards, ESPP contributions, and exercise of warrants to purchase common stock		8,873	1,161
Net cash provided by financing activities		8,873	249,316
Net increase in cash, cash equivalents and restricted cash		39,809	70,438
Cash, cash equivalents and restricted cash at beginning of period		239,274	145,649
Cash, cash equivalents and restricted cash at end of period		279,083	216,087
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Purchase of property and equipment included in accounts payable and accrued expenses		$ 425	$ 267
[1]	The Company has reclassified prior period amounts to conform with presentation as of March 31, 2020.

Nature of Business

Nature of Business	3 Months Ended
	Mar. 31, 2020
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, the need for substantial additional financing, the risk of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology, and compliance with government regulations.

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 , 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, 2019 , have not changed, and the unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2019 . 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 March 31, 2020 , the results of its operations for the three months ended March 31, 2020 and 2019 , and its cash flows for the three months ended March 31, 2020 and 2019 . 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 months ended March 31, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 , 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, 2019 , 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, 2019 .

Use of Estimates

Use of Estimates	3 Months Ended
	Mar. 31, 2020
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 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 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 consolidated financial statements, management used significant 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	3 Months Ended
	Mar. 31, 2020
Segment Reporting [Abstract]
Segment Information	Segment Information Operating 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

Cash Equivalents and Short-term

Cash Equivalents and Short-term and Long-term Investments	3 Months Ended
	Mar. 31, 2020
Cash and Cash Equivalents [Abstract]
Cash Equivalents and Short-term and Long-term Investments	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 March 31, 2020 as “available-for-sale” pursuant to ASC 320, Investments – Debt and Equity Securities. 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 in interest income interest and dividends on securities classified as available-for-sale. 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 as of March 31, 2020 (in thousands): Less than 12 months Fair Value Unrealized Losses Corporate obligations $ 50,840 $ (181 ) Total available-for sale securities in an unrealized loss position $ 50,840 $ (181 ) There were no securities in an unrealized loss for greater than 12 months as of March 31, 2020 . The unrealized losses on the Company's corporate obligations 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 March 31, 2020 . Prior to January 1, 2020, the Company reviewed marketable securities for other-than-temporary impairment whenever the fair value of a marketable security was less than the amortized cost and evidence indicated that a marketable security’s carrying amount was not recoverable within a reasonable period of time. Other-than-temporary impairments of investments were recognized in the consolidated statements of operations if the Company had experienced a credit loss, had the intent to sell the marketable security, or if it was more likely than not that the Company would be required to sell the marketable security before recovery of the amortized cost basis. Evidence considered in this assessment included reasons for the impairment, compliance with the Company’s investment policy, the severity and the duration of the impairment and changes in value subsequent to the end of the period. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2019 was $35.8 million . The aggregate fair value of securities held by the Company in an unrealized loss position for more than twelve months as of December 31, 2019 was zero . The aggregate unrealized loss for those securities in an unrealized loss position for more than twelve months was zero . The Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2019 . The following is a summary of cash, cash equivalents and available-for-sale securities as of March 31, 2020 and December 31, 2019 (in thousands): March 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 277,485 $ 1 $ — $ 277,486 Available-for-sale securities: Corporate obligations 97,260 56 (181 ) 97,135 U.S. Treasury securities 32,545 147 — 32,692 Certificates of deposit 245 3 — 248 Mortgage and other asset backed securities 8,000 2 — 8,002 Total available-for-sale securities $ 138,050 $ 208 $ (181 ) $ 138,077 Total cash, cash equivalents and available-for-sale securities $ 415,535 $ 209 $ (181 ) $ 415,563 December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 237,677 $ — $ — $ 237,677 Available-for-sale securities: Corporate obligations 124,676 219 (15 ) 124,880 U.S. Treasury securities 78,230 98 (1 ) 78,327 Certificates of deposit 490 3 — 493 Mortgage and other asset backed securities 12,476 5 (12 ) 12,469 Total available-for-sale securities $ 215,872 $ 325 $ (28 ) $ 216,169 Total cash, cash equivalents and available-for-sale securities $ 453,549 $ 325 $ (28 ) $ 453,846

Restricted Cash

Restricted Cash	3 Months Ended
	Mar. 31, 2020
Receivables [Abstract]
Restricted Cash	Restricted Cash The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statements of cash flows (in thousands): March 31, 2020 2019 Cash and cash equivalents $ 277,486 $ 214,490 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 279,083 $ 216,087 As of March 31, 2020 and December 31, 2019 , the Company maintained letters of credit totaling $1.6 million 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.

Concentrations of Credit Risk a

Concentrations of Credit Risk and Off-Balance Sheet Risk	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 .

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 and December 31, 2019 (in thousands): March 31, 2020 Quoted Prices in Active Markets for Identical Items (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 239,753 $ — $ — $ 239,753 Corporate obligations — 101,130 — 101,130 U.S. Treasury securities — 36,691 — 36,691 Certificates of deposit — 248 — 248 Mortgage and other asset backed securities — 8,002 — 8,002 Total assets $ 239,753 $ 146,071 $ — $ 385,824 Liabilities: Warrants to purchase common stock $ — $ — $ 3,304 $ 3,304 Total liabilities $ — $ — $ 3,304 $ 3,304 December 31, 2019 Quoted Prices in Active Markets for Identical Items (Level 1) Significant other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 193,867 $ — $ — $ 193,867 Corporate obligations — 138,369 — 138,369 U.S. Treasury securities — 83,819 — 83,819 Certificates of deposit — 493 — 493 Mortgage and other asset backed securities — 12,470 — 12,470 Total assets $ 193,867 $ 235,151 $ — $ 429,018 Liabilities: Warrants to purchase common stock $ — $ — $ 1,856 $ 1,856 Total liabilities $ — $ — $ 1,856 $ 1,856 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 three months ended March 31, 2020 or the year ended December 31, 2019 . Items measured at fair value on a recurring basis include short-term and long-term investments (Note 5), and warrants to purchase common stock (Note 12). During the periods presented, the Company has not changed the manner in which it values assets and liabilities that are measured at fair value using Level 3 inputs. The following table sets forth a summary of changes in the fair value of the Company’s common stock warrant liabilities, which represent a recurring measurement that is classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands): Three Months Ended March 31, 2020 2019 Beginning balance $ 1,856 $ 1,491 Change in fair value 1,448 116 Ending balance $ 3,304 $ 1,607 The fair value of the warrants to purchase common stock on the date of issuance and on each re-measurement date for those warrants classified as liabilities was estimated using either the Monte Carlo simulation framework, which incorporates future financing events over the remaining life of the warrants to purchase common stock, or for certain re-measurement dates, due to the warrants being deeply in the money, the Black-Scholes option pricing model. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement. At each reporting period, the Company evaluates the best valuation methodology. At March 31, 2020

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	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 Months Ended 2020 2019 Outstanding stock options 4,232 4,115 Common stock warrants 39 39 Shares issuable under employee stock purchase plan 13 13 Outstanding restricted stock units (1) 557 630 4,841 4,797 (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	3 Months Ended
	Mar. 31, 2020
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Comprehensive Loss	Comprehensive Loss Comprehensive 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 (loss) income, which includes certain changes in equity that are excluded from net loss. Comprehensive loss has been disclosed in the accompanying consolidated statements of operations and comprehensive loss. Accumulated other comprehensive (loss) income is presented separately on the consolidated balance sheets and consists entirely of unrealized holding gains and losses on investments as of March 31, 2020 and December 31, 2019 .

Recent Accounting Pronouncement

Recent Accounting Pronouncements	3 Months Ended
	Mar. 31, 2020
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses . The new standard requires entities to measure all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions and reasonable and supportable forecasts. For available-for-sale debt securities with unrealized losses, the standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The standard limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which the carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. On January 1, 2020 the Company adopted ASU 2016-13. For discussion regarding the impact of this accounting pronouncement and its amendments, refer to Note 5 within the notes to our consolidated financial statements appearing elsewhere in this Quarterly Report on Form 10-Q. In June 2018, the FASB issued ASU 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. This amendment aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. On January 1, 2020, the Company adopted ASU 2018-15 on a prospective basis, with no material impact on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes. The ASU simplifies the accounting for income taxes by eliminating certain exceptions to the guidance in ASC 740, Income Taxes , related to the approach for allocating income tax expense or benefit for the year to continuing operations, discontinued operations, other comprehensive income, and other charges or credits recorded directly to shareholders’ equity; the methodology for calculating income taxes in an interim period; and the recognition of deferred tax liabilities for outside basis differences. On January 1, 2020, the Company early adopted ASU 2019-12 on a prospective basis, with no material impact on its consolidated financial statements and related disclosures.

Warrants

Warrants	3 Months Ended
	Mar. 31, 2020
Warrants and Rights Note Disclosure [Abstract]
Warrants	Warrants Below is a summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants (in thousands, except per share data): Warrants as of Weighted- Average Exercise Balance Sheet Classification March 31, 2020 December 31, 2019 Price Per Share Expiration March 31, 2020 December 31, 2019 Warrants to purchase common stock 39 39 $ 5.88 June 10, 2020 - July 9, 2020 Liability Liability

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies 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 months ended March 31, 2020 , and, to the best of its knowledge, no material legal proceedings are currently pending or threatened. Other The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at March 31, 2020 and December 31, 2019 , or royalties on future sales of specified products. See Note 14 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	3 Months Ended
	Mar. 31, 2020
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 (the Restated Sotatercept Agreement) and clarified certain responsibilities of the Company and BMS in a letter agreement to the Restated Sotatercept Agreement on March 10, 2020. 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, 2019 , 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, 2019 . Accounting Analysis Upon adoption of ASC 606, all of the Company’s performance obligations pursuant to its arrangements with BMS have been completed and all remaining potential milestone payments have been fully constrained as they relate to future regulatory events that are outside of the Company’s control, and therefore the risk of significant reversal had not been resolved. As of March 31, 2020 , the next milestone payment for REBLOZYL would be $25.0 million and would result from European Medicines Agency (EMA) approval of a Biologics License Application (BLA) or equivalent for luspatercept-aamt in either myelodysplastic syndromes or beta-thalassemia. The Company expects the EMA to issue a decision on the MAA in the second half of 2020. 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 March 31, 2020 , the Company cannot determine if it is probable that a regulatory agency will approve the applications. Through March 31, 2020 , under all BMS arrangements the Company has received net cost-share payments and milestones of $186.2 million and $44.8 million for REBLOZYL and sotatercept, respectively. The Company recorded net revenue of $4.3 million and $2.8 million during the three months ended March 31, 2020 and 2019 , 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 also 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 under the licenses. During the three months ended March 31, 2020 and 2019 , the Company expensed milestones, fees and royalties totaling $0.3 million and zero , respectively. Milestones and fees associated with development related activities are recorded as research and development expense. 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 March 31, 2020 and 2019 , the Company expensed milestones and fees totaling zero and $0.2 million , respectively, 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. 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 March 31, 2020 and 2019 , the Company expensed milestones and fees totaling $0.3 million and zero , respectively, which is recorded as research and development expense.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended March 31, 2020 and 2019 , respectively, as follows (in thousands): Three Months Ended 2020 2019 Research and development $ 3,141 $ 3,500 Selling, general and administrative 3,538 3,492 $ 6,679 $ 6,992 Stock Options The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended 2020 2019 Expected volatility 55.1 % 59.1 % Expected term (in years) 6.0 6.0 Risk-free interest rate 1.6 % 2.6 % Expected dividend yield — % — % The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2020 (in thousands, except per share amounts and years): Number of Stock Options Weighted- Average Exercise Price Per Share Weighted- Average Contractual Life (in years) Aggregate Intrinsic Value(1) Outstanding at December 31, 2019 3,820 $ 36.26 6.81 Granted 835 $ 54.55 Exercised (296 ) $ 28.69 Canceled or forfeited (127 ) $ 41.03 Outstanding at March 31, 2020 4,232 $ 40.25 7.07 $ 210,031 Exercisable at March 31, 2020 2,328 $ 34.86 5.48 $ 128,039 (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 March 31, 2020 . The aggregate intrinsic value of options exercised during the three months ended March 31, 2020 was $15.0 million . As of March 31, 2020 , there was $44.6 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.84 years. Restricted Stock Units The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 397 $ 39.20 Granted 197 $ 54.31 Vested (86 ) $ 38.69 Forfeited (26 ) $ 41.36 Unvested balance at March 31, 2020 482 $ 45.36 As of March 31, 2020 , there was approximately $18.2 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 2.05 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 before December 31, 2022. As of March 31, 2020 , none of the PSU milestones had been achieved. When achievement of a milestone becomes probable, compensation cost will be recognized from the grant date over the requisite service period. As of March 31, 2020 , no related compensation cost had been recognized. The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 — $ — Granted (1) 78 $ 52.99 Vested — $ — Forfeited (3 ) $ 52.99 Unvested balance at March 31, 2020 75 $ 52.99 (1) Pursuant to the terms of the awards granted on January 22, 2020, the actual number of awards earned could range between 0% and 200% of the number of awards granted. As of March 31, 2020 , there was approximately $4.0 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 2.75 years . Depending on the actual number of awards earned, the actual expense recognized could range between 0% and 200% of this number.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions BMS BMS owned 11.8% and 12.0% of the Company’s fully diluted equity as of March 31, 2020 and December 31, 2019 , respectively. Refer to Note 14 for additional information regarding this collaboration arrangement. During the three months ended March 31, 2020 and 2019

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 , 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, 2019 , have not changed, and the unaudited interim financial statements have been prepared on the same basis as the audited annual financial statements as of and for the year ended December 31, 2019 . 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 March 31, 2020 , the results of its operations for the three months ended March 31, 2020 and 2019 , and its cash flows for the three months ended March 31, 2020 and 2019 . 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 months ended March 31, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 , 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, 2019 , 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, 2019 .
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 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 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 consolidated financial statements, management used significant 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.
Recent Accounting Pronouncements	In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses . The new standard requires entities to measure all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions and reasonable and supportable forecasts. For available-for-sale debt securities with unrealized losses, the standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The standard limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which the carrying value exceeds fair value and requires the reversal of previously recognized credit losses if fair value increases. On January 1, 2020 the Company adopted ASU 2016-13. For discussion regarding the impact of this accounting pronouncement and its amendments, refer to Note 5 within the notes to our consolidated financial statements appearing elsewhere in this Quarterly Report on Form 10-Q. In June 2018, the FASB issued ASU 2018-15, Intangible-Goodwill and Other Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. This amendment aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. On January 1, 2020, the Company adopted ASU 2018-15 on a prospective basis, with no material impact on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes. The ASU simplifies the accounting for income taxes by eliminating certain exceptions to the guidance in ASC 740, Income Taxes , related to the approach for allocating income tax expense or benefit for the year to continuing operations, discontinued operations, other comprehensive income, and other charges or credits recorded directly to shareholders’ equity; the methodology for calculating income taxes in an interim period; and the recognition of deferred tax liabilities for outside basis differences. On January 1, 2020, the Company early adopted ASU 2019-12 on a prospective basis, with no material impact on its consolidated financial statements and related disclosures.

Cash Equivalents and Short-te_2

Cash Equivalents and Short-term and Long-term Investments (Tables)	3 Months Ended
	Mar. 31, 2020
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 as of March 31, 2020 (in thousands): Less than 12 months Fair Value Unrealized Losses Corporate obligations $ 50,840 $ (181 ) Total available-for sale securities in an unrealized loss position $ 50,840 $ (181 )
Available-for-sale securities	The following is a summary of cash, cash equivalents and available-for-sale securities as of March 31, 2020 and December 31, 2019 (in thousands): March 31, 2020 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 277,485 $ 1 $ — $ 277,486 Available-for-sale securities: Corporate obligations 97,260 56 (181 ) 97,135 U.S. Treasury securities 32,545 147 — 32,692 Certificates of deposit 245 3 — 248 Mortgage and other asset backed securities 8,000 2 — 8,002 Total available-for-sale securities $ 138,050 $ 208 $ (181 ) $ 138,077 Total cash, cash equivalents and available-for-sale securities $ 415,535 $ 209 $ (181 ) $ 415,563 December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and cash equivalents due in 90 days or less $ 237,677 $ — $ — $ 237,677 Available-for-sale securities: Corporate obligations 124,676 219 (15 ) 124,880 U.S. Treasury securities 78,230 98 (1 ) 78,327 Certificates of deposit 490 3 — 493 Mortgage and other asset backed securities 12,476 5 (12 ) 12,469 Total available-for-sale securities $ 215,872 $ 325 $ (28 ) $ 216,169 Total cash, cash equivalents and available-for-sale securities $ 453,549 $ 325 $ (28 ) $ 453,846

Restricted Cash (Tables)

Restricted Cash (Tables)	3 Months Ended
	Mar. 31, 2020
Receivables [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 sheet that sum to the total of the same such amounts shown in the statements of cash flows (in thousands): March 31, 2020 2019 Cash and cash equivalents $ 277,486 $ 214,490 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 279,083 $ 216,087
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 sheet that sum to the total of the same such amounts shown in the statements of cash flows (in thousands): March 31, 2020 2019 Cash and cash equivalents $ 277,486 $ 214,490 Restricted cash 1,597 1,597 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 279,083 $ 216,087

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2020
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 March 31, 2020 and December 31, 2019 (in thousands): March 31, 2020 Quoted Prices in Active Markets for Identical Items (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 239,753 $ — $ — $ 239,753 Corporate obligations — 101,130 — 101,130 U.S. Treasury securities — 36,691 — 36,691 Certificates of deposit — 248 — 248 Mortgage and other asset backed securities — 8,002 — 8,002 Total assets $ 239,753 $ 146,071 $ — $ 385,824 Liabilities: Warrants to purchase common stock $ — $ — $ 3,304 $ 3,304 Total liabilities $ — $ — $ 3,304 $ 3,304 December 31, 2019 Quoted Prices in Active Markets for Identical Items (Level 1) Significant other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets: Money market funds $ 193,867 $ — $ — $ 193,867 Corporate obligations — 138,369 — 138,369 U.S. Treasury securities — 83,819 — 83,819 Certificates of deposit — 493 — 493 Mortgage and other asset backed securities — 12,470 — 12,470 Total assets $ 193,867 $ 235,151 $ — $ 429,018 Liabilities: Warrants to purchase common stock $ — $ — $ 1,856 $ 1,856 Total liabilities $ — $ — $ 1,856 $ 1,856
Summary of changes in the fair value of the preferred and common stock warrant liability	The following table sets forth a summary of changes in the fair value of the Company’s common stock warrant liabilities, which represent a recurring measurement that is classified within Level 3 of the fair value hierarchy, wherein fair value is estimated using significant unobservable inputs (in thousands): Three Months Ended March 31, 2020 2019 Beginning balance $ 1,856 $ 1,491 Change in fair value 1,448 116 Ending balance $ 3,304 $ 1,607

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2020
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 Months Ended 2020 2019 Outstanding stock options 4,232 4,115 Common stock warrants 39 39 Shares issuable under employee stock purchase plan 13 13 Outstanding restricted stock units (1) 557 630 4,841 4,797 (1) This balance is comprised of both the restricted stock units and performance-based restricted stock units described in Note 15.

Warrants (Tables)

Warrants (Tables)	3 Months Ended
	Mar. 31, 2020
Warrants and Rights Note Disclosure [Abstract]
Summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants	Below is a summary of the number of shares issuable upon exercise of outstanding warrants and the terms and accounting treatment for the outstanding warrants (in thousands, except per share data): Warrants as of Weighted- Average Exercise Balance Sheet Classification March 31, 2020 December 31, 2019 Price Per Share Expiration March 31, 2020 December 31, 2019 Warrants to purchase common stock 39 39 $ 5.88 June 10, 2020 - July 9, 2020 Liability Liability

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2020
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 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended March 31, 2020 and 2019 , respectively, as follows (in thousands): Three Months Ended 2020 2019 Research and development $ 3,141 $ 3,500 Selling, general and administrative 3,538 3,492 $ 6,679 $ 6,992
Schedule of weighted-average assumptions used for estimating fair value	The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended 2020 2019 Expected volatility 55.1 % 59.1 % Expected term (in years) 6.0 6.0 Risk-free interest rate 1.6 % 2.6 % Expected dividend yield — % — %
Summary of stock option activity	The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2020 (in thousands, except per share amounts and years): Number of Stock Options Weighted- Average Exercise Price Per Share Weighted- Average Contractual Life (in years) Aggregate Intrinsic Value(1) Outstanding at December 31, 2019 3,820 $ 36.26 6.81 Granted 835 $ 54.55 Exercised (296 ) $ 28.69 Canceled or forfeited (127 ) $ 41.03 Outstanding at March 31, 2020 4,232 $ 40.25 7.07 $ 210,031 Exercisable at March 31, 2020 2,328 $ 34.86 5.48 $ 128,039 (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 March 31, 2020 .
Schedule of nonvested restricted stock units activity	The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 397 $ 39.20 Granted 197 $ 54.31 Vested (86 ) $ 38.69 Forfeited (26 ) $ 41.36 Unvested balance at March 31, 2020 482 $ 45.36
Share-based compensation, performance shares award nonvested activity	The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2020 (in thousands, except per share amounts): Number Weighted- Unvested balance at December 31, 2019 — $ — Granted (1) 78 $ 52.99 Vested — $ — Forfeited (3 ) $ 52.99 Unvested balance at March 31, 2020 75 $ 52.99 (1) Pursuant to the terms of the awards granted on January 22, 2020, the actual number of awards earned could range between 0% and 200% of the number of awards granted.

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 2020segment
Segment Information
Number of operating segments	1

Cash Equivalents and Short-te_3

Cash Equivalents and Short-term and Long-term Investments Summary of Available-for-sale Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 50,840
Unrealized Losses	(181)	$ (35,800)
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	50,840
Unrealized Losses	$ (181)

Cash Equivalents and Short-te_4

Cash Equivalents and Short-term and Long-term Investments - Narrative (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Abstract]
Unrealized loss position, less than twelve months, fair value	$ 181,000	$ 35,800,000
Continuous unrealized loss position, twelve months or longer, fair value		0
Continuous unrealized loss position, fair value		$ 0

Cash Equivalents and Short-te_5

Cash Equivalents and Short-term and Long-term Investments - Summary of Cash, Cash Equivalents and Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Cash and cash equivalents due in 90 days or less
Amortized Cost	$ 277,485	$ 237,677
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Estimated Fair Value	277,486	237,677	$ 214,490
Available-for-sale securities:
Amortized Cost	138,050	215,872
Gross Unrealized Gains	208	325
Gross Unrealized Losses	(181)	(28)
Estimated Fair Value	138,077	216,169
Total cash, cash equivalents and available-for-sale securities
Amortized Cost	415,535	453,549
Gross Unrealized Gains	209	325
Gross Unrealized Losses	(181)	(28)
Estimated Fair Value	415,563	453,846
Corporate obligations
Available-for-sale securities:
Amortized Cost	97,260	124,676
Gross Unrealized Gains	56	219
Gross Unrealized Losses	(181)	(15)
Estimated Fair Value	97,135	124,880
U.S. Treasury securities
Available-for-sale securities:
Amortized Cost	32,545	78,230
Gross Unrealized Gains	147	98
Gross Unrealized Losses	0	(1)
Estimated Fair Value	32,692	78,327
Certificates of deposit
Available-for-sale securities:
Amortized Cost	245	490
Gross Unrealized Gains	3	3
Gross Unrealized Losses	0	0
Estimated Fair Value	248	493
Mortgage and other asset backed securities
Available-for-sale securities:
Amortized Cost	8,000	12,476
Gross Unrealized Gains	2	5
Gross Unrealized Losses	0	(12)
Estimated Fair Value	$ 8,002	$ 12,469

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	$ 277,486	$ 237,677	$ 214,490
Restricted cash	1,597		1,597
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	279,083	239,274	$ 216,087	$ 145,649
Certificates of Deposits and Money Market Funds | Letter of Credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 1,600	$ 1,600

Concentrations of Credit Risk_2

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

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Total assets	$ 385,824	$ 429,018
Liabilities:
Total liabilities	3,304	1,856
Common stock warrants
Liabilities:
Total liabilities	3,304	1,856
Money market funds
Assets:
Total assets	239,753	193,867
Corporate obligations
Assets:
Total assets	101,130	138,369
U.S. Treasury securities
Assets:
Total assets	36,691	83,819
Certificates of deposit
Assets:
Total assets	248	493
Mortgage and other asset backed securities
Assets:
Total assets	8,002	12,470
Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	239,753	193,867
Liabilities:
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Common stock warrants
Liabilities:
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Total assets	239,753	193,867
Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Total assets	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Total assets	146,071	235,151
Liabilities:
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Common stock warrants
Liabilities:
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Total assets	0	0
Significant Other Observable Inputs (Level 2) | Corporate obligations
Assets:
Total assets	101,130	138,369
Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Total assets	36,691	83,819
Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Total assets	248	493
Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Total assets	8,002	12,470
Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Liabilities:
Total liabilities	3,304	1,856
Significant Unobservable Inputs (Level 3) | Common stock warrants
Liabilities:
Total liabilities	3,304	1,856
Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Total assets	$ 0	$ 0

Fair Value Measurements - Prefe

Fair Value Measurements - Preferred and Common Stock Warrant Liability Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Summary of changes in the fair value of the preferred and common stock warrant liability classified within Level 3 of the fair value hierarchy
Beginning balance	$ 1,856	$ 1,491
Change in fair value	1,448	116
Ending balance	$ 3,304	$ 1,607

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Income (Loss) Per Share
Anti-dilutive common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	4,841	4,797
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)	4,232	4,115
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)	39	39
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)	13	13
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)	557	630

Warrants (Details)

Warrants (Details) - Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020 - $ / shares shares in Thousands	Mar. 31, 2020	Dec. 31, 2019
Warrants
Warrants (in shares)	39	39
Weighted-average exercise price per share (in usd per share)	$ 5.88

Significant Agreements - Celgen

Significant Agreements - Celgene, Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Significant Agreements
Revenue from contract with customer	$ 4,344	$ 2,780
Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received	186,200
Sotarcept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received	44,800
Cost-sharing, net
Significant Agreements
Revenue from contract with customer	2,824	2,780
Cost-sharing, net | Collaboration, License, and Option Agreement
Significant Agreements
Revenue from contract with customer	4,300	$ 2,800
Regulatory Milestone | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 25,000

Significant Agreements - Other

Significant Agreements - Other Agreements (Details) - USD ($)	1 Months Ended		3 Months Ended		12 Months Ended
	Dec. 31, 2019	May 31, 2014	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2004
Significant Agreements
Milestones and fees			$ 37,663,000	$ 32,771,000
License agreement with antibody technology company
Significant Agreements
Milestones and fees			0	200,000
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			300,000	0
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			$ 300,000	$ 0
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 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%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Costs Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 6,679	$ 6,992
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	3,141	3,500
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost
Stock-based compensation expense	$ 3,538	$ 3,492

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used for Estimating Fair Value (Details) - Stock Option	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Expected volatility	55.10%	59.10%
Expected term (in years)	6 years	6 years
Risk-free interest rate	1.60%	2.60%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Number of Stock Options
Outstanding at the beginning of period (in shares)	3,820
Granted (in shares)	835
Exercised (in shares)	(296)
Canceled or forfeited (in shares)	(127)
Outstanding at the end of the period (in shares)	4,232	3,820
Exercisable at the end of the period (in shares)	2,328
Weighted- Average Exercise Price Per Share
Outstanding at the beginning of period (in usd per share)	$ 36.26
Granted (in usd per share)	54.55
Exercised (in usd per share)	28.69
Canceled or forfeited (in usd per share)	41.03
Outstanding at the end of the period (in usd per share)	40.25	$ 36.26
Exercisable at the end of the period (in usd per share)	$ 34.86
Weighted- Average Contractual Life (in years)
Outstanding at the beginning of the period	7 years 25 days	6 years 9 months 21 days
Outstanding at the end of the period	7 years 25 days	6 years 9 months 21 days
Exercisable at the end of the period	5 years 5 months 23 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 210,031
Exercisable at the end of the period	128,039
Aggregate intrinsic value of options exercised	15,000
Unrecognized compensation expense	$ 44,600
Weighted-average period for recognition	2 years 10 months 2 days

Stock-Based Compensation - Sc_3

Stock-Based Compensation - Schedule of Nonvested Restricted Stock Activity (Details) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)$ / sharesshares
Weighted- Average Grant Date Fair Value
Weighted-average period for recognition	2 years 10 months 2 days
Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	397
Granted (in shares) | shares	197
Vested (in shares) | shares	(86)
Forfeited (in shares) | shares	(26)
Unvested balance at end of the period (in shares) | shares	482
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 39.20
Granted (in usd per share) | $ / shares	54.31
Vested (in usd per share) | $ / shares	38.69
Forfeited (in usd per share) | $ / shares	41.36
Unvested balance at end of the period (in usd per share) | $ / shares	$ 45.36
Unrecognized compensation cost | $	$ 18.2
Weighted-average period for recognition	2 years 18 days
Performance-Based Restricted Stock Units
Number of Grants
Unvested balance at beginning of the period (in shares) | shares	0
Granted (in shares) | shares	78
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(3)
Unvested balance at end of the period (in shares) | shares	75
Weighted- Average Grant Date Fair Value
Unvested balance at beginning of the period (in usd per share) | $ / shares	$ 0
Granted (in usd per share) | $ / shares	52.99
Vested (in usd per share) | $ / shares	0
Forfeited (in usd per share) | $ / shares	52.99
Unvested balance at end of the period (in usd per share) | $ / shares	$ 52.99
Unrecognized compensation cost | $	$ 4
Weighted-average period for recognition	2 years 9 months
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%

Related Party Transactions (Det

Related Party Transactions (Details)	Mar. 31, 2020	Dec. 31, 2019
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	11.80%	12.00%