Acceleron Pharma (XLRN) 10-K17 Mar 142013 FY Annual reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Jan. 31, 2014
Document and Entity Information	'	'
Entity Registrant Name	'ACCELERON PHARMA INC	'
Entity Central Index Key	'0001280600	'
Document Type	'10-K	'
Document Period End Date	31-Dec-13	'
Amendment Flag	'false	'
Current Fiscal Year End Date	'--12-31	'
Entity Well-known Seasoned Issuer	'No	'
Entity Voluntary Filers	'No	'
Entity Current Reporting Status	'Yes	'
Entity Filer Category	'Non-accelerated Filer	'
Entity Public Float	$559	'
Entity Common Stock, Shares Outstanding	'	31,160,274
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'FY	'

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Current assets:	'	'
Cash and cash equivalents	$113,163	$39,611
Collaboration receivables (includes related party amounts of $3,616 and $1,840 at December 31, 2013 and 2012, respectively)	3,616	2,776
Prepaid expenses and other current assets	2,243	1,474
Total current assets	119,022	43,861
Property and equipment, net	3,705	4,059
Restricted cash	913	913
Related party receivables	'	233
Other assets	92	146
Total assets	123,732	49,212
Current liabilities:	'	'
Accounts payable	885	642
Accrued expenses (includes related party amounts of $0 and $861at December 31, 2013 and 2012, respectively)	6,927	6,153
Deferred revenue	2,031	27,840
Deferred rent	499	499
Notes payable, net of discount	16,868	3,668
Total current liabilities	27,210	38,802
Deferred revenue, net of current portion	5,620	6,760
Deferred rent, net of current portion	2,337	2,837
Notes payable, net of current portion and discount	'	16,525
Warrants to purchase redeemable convertible preferred stock	'	1,422
Warrants to purchase common stock	30,753	5,229
Total liabilities	65,920	71,575
Commitments and contingencies (Note 7)	'	'
Redeemable convertible preferred stock (Note 8)	'	268,610
Stockholders' equity (deficit):	'	'
Undesignated preferred stock, $0.001 par value: 25,000,000 shares authorized and no shares issued or outstanding at December 31, 2013; No shares authorized, issued or outstanding at December 31, 2012	'	'
Common stock, $0.001 par value: 175,000,000 and 104,013,161 shares authorized at December 31, 2013 and 2012, respectively; 28,348,630 and 2,432,155, shares issued and outstanding at December 31, 2013 and 2012, respectively	29	3
Additional paid-in capital	250,107	'
Accumulated deficit	-192,324	-290,976
Total stockholders' equity (deficit)	57,812	-290,973
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$123,732	$49,212

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Consolidated Balance Sheets	'	'
Collaboration receivables from related party (in dollars)	$3,616	$1,840
Accrued expenses due to related party (in dollars)	$0	$861
Preferred stock, par value (in dollars per share)	$0.00	$0.00
Undesignated preferred stock shares authorized	25,000,000	'
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$0.00	$0.00
Common stock, shares authorized	175,000,000	104,013,161
Common stock, shares issued	28,348,630	2,432,155
Common stock, shares outstanding	28,348,630	2,432,155

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive (Loss) Income (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Collaboration revenue:	'	'	'
License and milestone	$43,948	$9,696	$74,406
Cost-sharing, net	13,282	5,558	4,760
Contract manufacturing	'	'	1,745
Total revenue	57,230	15,254	80,911
Costs and expenses:	'	'	'
Research and development	36,051	35,319	32,713
General and administrative	14,227	8,824	8,142
Cost of contract manufacturing revenue	'	'	1,500
Total costs and expenses	50,278	44,143	42,355
Income (loss) from operations	6,952	-28,889	38,556
Other (expense) income:	'	'	'
Other expense, net	-26,797	-2,255	-485
Interest income	39	91	17
Interest expense	-2,092	-1,529	-1,822
Total other expense, net	-28,850	-3,693	-2,290
Net (loss) income	-21,898	-32,582	36,266
Comprehensive (loss) income	-21,898	-32,582	36,266
Reconciliation of net (loss) income to net (loss) income applicable to common stockholders:	'	'	'
Net (loss) income	-21,898	-32,582	36,266
Accretion of dividends, interest, redemption value and issuance costs on redeemable convertible preferred stock	-19,870	-27,061	-21,757
Gain on extinguishment of redeemable convertible preferred stock	2,765	'	'
Net (loss) income applicable to participating securities	'	'	-12,645
Net (loss) income applicable to common stockholders-basic	-39,003	-59,643	1,864
Net (loss) income	-21,898	-32,582	36,266
Accretion of dividends, interest, redemption value and issuance costs on redeemable convertible preferred stock	-19,870	-27,061	-21,757
Gain on extinguishment of redeemable convertible preferred stock	2,765	'	'
Net (loss) income applicable to participating securities	'	'	-12,395
Net (loss) income applicable to common stockholders-diluted	($39,003)	($59,643)	$2,114
Net (loss) income per share applicable to common stockholders: (Note 2)	'	'	'
Basic (in dollars per share)	($4.15)	($24.84)	$0.80
Diluted (in dollars per share)	($4.15)	($24.84)	$0.78
Weighted-average number of common shares used in computing net (loss) income per share applicable to common stockholders:	'	'	'
Basic (in shares)	9,407	2,401	2,328
Diluted (in shares)	9,407	2,401	2,716

Consolidated_Statements_of_Ope1

Consolidated Statements of Operations and Comprehensive (Loss) Income (Parenthetical) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Consolidated Statements of Operations and Comprehensive (Loss) Income	'	'	'
Related party revenue	$32,284	$4,914	$64,220

Consolidated_Statements_of_Red

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $)	Total	Redeemable Convertible Preferred Stock	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series C Redeemable Convertible Preferred Stock	Series C-1 Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock	Series D-1 Redeemable Convertible Preferred Stock	Series E Redeemable Convertible Preferred Stock	Series F Redeemable Convertible Preferred Stock
In Thousands, except Share data, unless otherwise specified	Total	Redeemable Convertible Preferred Stock	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series C Redeemable Convertible Preferred Stock	Series C-1 Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock	Series D-1 Redeemable Convertible Preferred Stock	Series E Redeemable Convertible Preferred Stock	Series F Redeemable Convertible Preferred Stock
Balance at Dec. 31, 2010	'	$189,414	$57,433	$55,880	$48,726	$7,571	$2,989	$8,392	$8,423	'
Balance (in shares) at Dec. 31, 2010	'	'	6,410,976	4,204,185	2,978,062	457,875	234,940	636,942	816,060	'
Increase (decrease) in temporary equity	'	'	'	'	'	'	'	'	'	'
Sale of Series F redeemable convertible preferred stock net of issuance costs of $92	'	30,378	'	'	'	'	'	'	'	30,378
Sale of Series F redeemable convertible preferred stock net of issuance costs of $92 (in shares)	'	'	'	'	'	'	'	'	'	2,426,171
Accretion of dividends, interest, redemption value and issuance costs related to redeemable convertible preferred stock	-21,757	21,757	4,616	5,584	5,594	908	668	1,736	2,511	140
Balance at Dec. 31, 2011	'	241,549	62,049	61,464	54,320	8,479	3,657	10,128	10,934	30,518
Balance (in shares) at Dec. 31, 2011	'	'	6,410,976	4,204,185	2,978,062	457,875	234,940	636,942	816,060	2,426,171
Increase (decrease) in temporary equity	'	'	'	'	'	'	'	'	'	'
Accretion of dividends, interest, redemption value and issuance costs related to redeemable convertible preferred stock	-27,061	27,061	4,616	5,580	5,589	908	668	1,736	2,459	5,505
Balance at Dec. 31, 2012	268,610	268,610	66,665	67,044	59,909	9,387	4,325	11,864	13,393	36,023
Balance (in shares) at Dec. 31, 2012	'	'	6,410,976	4,204,185	2,978,062	457,875	234,940	636,942	816,060	2,426,171
Increase (decrease) in temporary equity	'	'	'	'	'	'	'	'	'	'
Accretion of dividends, interest, redemption value and issuance costs related to redeemable convertible preferred stock	-19,870	19,870	3,408	4,138	4,068	661	487	1,273	1,802	4,033
Repurchase and retirement of redeemable convertible preferred stock	'	-3,064	'	-2,267	-445	'	-54	'	-224	-74
Repurchase and retirement of redeemable convertible preferred stock (in shares)	'	'	'	-139,741	-21,744	'	-2,906	'	-13,103	-4,825
Exercise of warrants to purchase convertible preferred stock	'	678	678	'	'	'	'	'	'	'
Exercise of warrants to purchase convertible preferred stock (in shares)	'	'	46,668	'	'	'	'	'	'	'
Conversion of redeemable convertible preferred stock into common stock	'	($286,094)	($70,751)	($68,915)	($63,532)	($10,048)	($4,758)	($13,137)	($14,971)	($39,982)
Conversion of redeemable convertible preferred stock into common stock (in shares)	'	'	-6,457,644	-4,064,444	-2,956,318	-457,875	-232,034	-636,942	-802,957	-2,421,346
Balance at Dec. 31, 2013	'	'	'	'	'	'	'	'	'	'

Consolidated_Statements_of_Red1

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $)	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit
In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit
Balance at Dec. 31, 2010	($248,817)	$3	'	($248,820)
Balance (in shares) at Dec. 31, 2010	'	2,261,461	'	'
Increase (Decrease) in Stockholders' Equity	'	'	'	'
Accretion of dividends, interest, redemption value and issuance costs related to redeemable convertible preferred stock	-21,757	'	-1,617	-20,140
Exercise of warrants to purchase common stock	'	'	0	'
Exercise of warrants to purchase common stock (in shares)	'	37,249	'	'
Compensation expense associated with stock options	1,212	'	1,212	'
Grant of stock options to nonemployees	215	'	215	'
Exercise of stock options	190	'	190	'
Exercise of stock options (in shares)	'	94,748	'	'
Net (loss) income	36,266	'	'	36,266
Balance at Dec. 31, 2011	-232,691	3	'	-232,694
Balance (in shares) at Dec. 31, 2011	'	2,393,458	'	'
Increase (Decrease) in Stockholders' Equity	'	'	'	'
Accretion of dividends, interest, redemption value and issuance costs related to redeemable convertible preferred stock	-27,061	'	-1,361	-25,700
Compensation expense associated with stock options	1,206	'	1,206	'
Exercise of stock options	155	'	155	'
Exercise of stock options (in shares)	'	38,697	'	'
Net (loss) income	-32,582	'	'	-32,582
Balance at Dec. 31, 2012	-290,973	3	'	-290,976
Balance (in shares) at Dec. 31, 2012	'	2,432,155	'	'
Increase (Decrease) in Stockholders' Equity	'	'	'	'
Accretion of dividends, interest, redemption value and issuance costs related to redeemable convertible preferred stock	-19,870	'	-4,230	-15,640
Repurchase and retirement of redeemable convertible preferred stock	2,773	'	2,773	'
Repurchase and retirement of redeemable convertible preferred stock (in shares)	'	-722	'	'
Exercise of warrants to purchase common stock (in shares)	'	23,735	'	'
Compensation expense associated with stock options	2,196	'	2,196	'
Exercise of stock options	653	'	653	'
Exercise of stock options (in shares)	'	292,802	'	'
Conversion of redeemable convertible preferred stock into common stock	286,094	19	149,885	136,190
Conversion of redeemable convertible preferred stock into common stock (in shares)	'	18,516,993	'	'
Reclassification of warrants to purchase shares of redeemable convertible preferred stock into warrants to purchase common stock	2,012	'	2,012	'
Issuance of common stock in connection with initial public offering and private placement, net of issuance costs of $2,692	96,825	7	96,818	'
Issuance of common stock in connection with initial public offering and private placement, net of issuance costs of $2,692 (in shares)	'	7,083,667	'	'
Net (loss) income	-21,898	'	'	-21,898
Balance at Dec. 31, 2013	$57,812	$29	$250,107	($192,324)
Balance (in shares) at Dec. 31, 2013	'	28,348,630	'	'

Consolidated_Statements_of_Red2

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2011
	Common Stock	Series F Redeemable Convertible Preferred Stock
Issuance costs	$2,692	$92

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Activities	'	'	'
Net (loss) income	($21,898)	($32,582)	$36,266
Adjustments to reconcile net (loss) income to net cash (used in) provided by operating activities:	'	'	'
Depreciation and amortization	915	1,293	3,134
Loss on disposition of property and equipment	32	'	'
Stock-based compensation	2,196	1,206	1,427
Amortization of debt discount	'	51	162
Accretion of deferred interest	342	335	271
Amortization of deferred debt issuance costs	186	84	79
Change in fair value of warrants	26,875	2,258	481
Gain on retirement of warrants	-76	'	'
Forgiveness of related party receivable	237	'	'
Changes in assets and liabilities:	'	'	'
Prepaid expenses and other current assets	-823	-594	2,564
Collaboration receivables	-840	-1,116	1,836
Related party receivable	-4	-8	-6
Accounts payable	-53	-1,272	334
Accrued expenses	709	1,640	-2,773
Deferred revenue	-26,949	-9,696	-35,130
Deferred rent	-499	-482	211
Restricted cash	'	-1	200
Net cash (used in) provided by operating activities	-19,650	-38,884	9,056
Investing Activities	'	'	'
Purchases of property and equipment	-307	-441	-27
Net cash used in investing activities	-307	-441	-27
Financing Activities	'	'	'
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	'	'	30,378
Proceeds from long-term debt, net of issuance costs	'	19,935	'
Proceeds from issuance of common stock from initial public offering, net of issuance costs	86,825	'	'
Proceeds from issuance of common stock from private placements	10,000	'	'
Payments of long-term debt	-3,669	-6,191	-9,476
Payments made to repurchase redeemable convertible preferred stock, common stock and warrants to purchase common stock	-300	'	'
Proceeds from exercise of stock options and warrants to purchase common stock	653	155	190
Net cash provided by financing activities	93,509	13,899	21,092
Net increase (decrease) in cash and cash equivalents	73,552	-25,426	30,121
Cash and cash equivalents at beginning of period	39,611	65,037	34,916
Cash and cash equivalents at end of period	113,163	39,611	65,037
Supplemental Disclosure of Cash Flow Information:	'	'	'
Cash paid for interest	1,657	1,065	1,461
Supplemental Disclosure of Non-Cash Investing and Financing Activities:	'	'	'
Accretion of dividends, interest, redemption value, and issuance costs on preferred stock	19,870	27,061	21,757
Conversion of preferred stock into common stock	286,094	'	'
Conversion of preferred stock warrants into common stock warrants	2,012	'	'
Cashless exercise of warrants	678	'	'
Capitalized follow-on public offering costs included in accrued expenses	74	'	'
Purchase of property and equipment included in accounts payable and accrued expenses	$297	'	'

Nature_of_Business

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2013
Nature of Business	'
Nature of Business	'

	1. Nature of Business

	Acceleron Pharma Inc. (Acceleron or the Company) was incorporated in the state of Delaware on June 13, 2003, as Phoenix Pharma, Inc. The Company subsequently changed its name to Acceleron Pharma Inc. and commenced operations in February 2004. The Company is a Cambridge, Massachusetts-based biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. The Company's research focuses on the biology of the Transforming Growth Factor-Beta (TGF-b) protein superfamily, a large and diverse group of molecules that regulate the growth and repair of tissues throughout the human body. By coupling its discovery and development expertise, including its proprietary knowledge of the TGF-b superfamily, with internal protein engineering and manufacturing capabilities, the Company has built a highly productive research and development platform that has generated numerous innovative protein therapeutics with novel mechanisms of action. The Company has internally discovered three protein therapeutics that are currently being studied in 12 ongoing Phase 2 clinical trials, focused on the areas of cancer and rare diseases.

	The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, risk that the Company never achieves profitability, the need for substantial additional financing, risk of relying on third parties, risks of clinical trial failures, dependence on key personnel, protection of proprietary technology and compliance with government regulations.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2013
Summary of Significant Accounting Policies	'
Summary of Significant Accounting Policies	'

	2. Summary of Significant Accounting Policies

	The accompanying consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the consolidated financial statements. The Company believes that a significant accounting policy is one that is both important to the portrayal of the Company's financial condition and results, and requires management's most difficult, subjective, or complex judgments, often as the result of the need to make estimates about the effect of matters that are inherently uncertain.

	Principles of Consolidation

	The accompanying consolidated financial statements include those of the Company and its wholly-owned subsidiary, Acceleron Pharma Security Corporation. All significant intercompany balances and transactions have been eliminated in consolidation.

	Basis of Presentation

	The accompanying 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).

	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. Actual results could materially differ from those estimates.

	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: revenue recognition, stock-based compensation expense, the determination of the fair value of stock-based awards, the fair value of liability-classified warrants, accrued expenses, and the recoverability of the Company's net deferred tax assets and related valuation allowance.

	The Company utilized significant estimates and assumptions in determining the fair value of its common stock, prior to the completion of its IPO. The Company's board of directors (the Board) determined the estimated fair value of the Company's common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector and the prices at which the Company sold shares of redeemable convertible preferred stock, the superior rights and preferences of securities senior to the Company's common stock at the time, and the likelihood of achieving a liquidity event, such as an IPO or sale of the Company.

	Collaboration Receivable

	Credit is extended to customers based upon an evaluation of the customer's financial condition. Collaboration receivables are recorded at net realizable value. The Company does not charge interest on past due balances. Collaboration receivables are determined to be past due when the payment due date is exceeded. The Company utilizes a specific identification accounts receivable reserve methodology based on a review of outstanding balances and previous activities to determine the allowance for doubtful accounts. The Company charges off uncollectible receivables at the time the Company determines the receivable is no longer collectible. The Company did not have an allowance for doubtful accounts at December 31, 2013 or 2012.

	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 operating segment, which is the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. All material long-lived assets of the Company reside in the United States. The Company does use contract research organizations (CROs) and research institutions located outside the United States. Some of these expenses are subject to collaboration reimbursement which is presented as a component of cost sharing, net in the statement of operations and comprehensive income (loss).

	Cash and Cash Equivalents

	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 market value.

	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 and accounts receivable. The Company maintains its cash and cash equivalent balances in the form of money market accounts with financial institutions that management believes are creditworthy. 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 concentration of credit risk.

	The Company routinely assesses the creditworthiness of its customers and collaboration partners. 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 additional credit risk beyond amounts provided for collection losses is believed by management to be probable in the Company's accounts receivable.

	Disclosure of Fair Value of Financial Instruments

	The carrying amounts of the Company's financial instruments, which include cash, cash equivalents, collaboration receivables, accounts payable, accrued expenses and notes payable, approximated their fair values at December 31, 2013 and 2012, due to the short-term nature of these instruments, and for the notes payable, the interest rates the Company believes it could obtain for borrowings with similar terms. See discussion below on the determination of the fair value of the Company's preferred and common stock warrants.

	The Company has evaluated the estimated fair value of financial instruments using available market information and management's estimates. The use of different market assumptions and/or estimation methodologies could have a significant effect on the estimated fair value amounts.

	Fair Value Measurements

	ASC Topic 820, Fair Value Measurement (ASC 820), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company's own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company's assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

	ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC Topic 820 establishes a three-tier fair value hierarchy that distinguishes between the following:



	•
	Level 1—Quoted market prices in active markets for identical assets or liabilities.

	•
	Level 2—Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves.

	•
	Level 3—Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

	To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument's level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Items measured at fair value on a recurring basis include warrants to purchase redeemable convertible preferred stock, which were outstanding until the closing of the IPO, and warrants to purchase common stock (Note 6). 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 tables set forth the Company's financial instruments carried at fair value using the lowest level of input applicable to each financial instrument as of December 31, 2013 and 2012 (in thousands):


		December 31, 2013
		Quoted Prices			Significant Other		Significant		Total
		in Active Markets			Observable		Unobservable
		for Identical Items			Inputs		Inputs
		(Level 1)			(Level 2)		(Level 3)
Assets:
Money market funds		$	101,394		$	—	$	—	$	101,394
Restricted cash			913			—		—		913




Total assets		$	102,307		$	—	$	—	$	102,307




Liabilities:
Warrants to purchase common stock		$	—		$	—	$	30,753	$	30,753


Total liabilities		$	—		$	—	$	30,753	$	30,753









		December 31, 2012
		Quoted Prices			Significant other		Significant		Total
		in Active Markets			Observable		Unobservable
		for Identical Items			Inputs		Inputs
		(Level 1)			(Level 2)		(Level 3)
Assets:
Money market funds		$	36,847		$	—	$	—	$	36,847
Restricted cash			913			—		—		913




Total assets		$	37,760		$	—	$	—	$	37,760




Liabilities:
Warrants to purchase redeemable convertible preferred stock		$	—		$	—	$	1,422	$	1,422
Warrants to purchase common stock			—			—		5,229		5,229


Total liabilities		$	—		$	—	$	6,651	$	6,651





The following table sets forth a summary of changes in the fair value of the Company's preferred and 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):


		Year Ended
		December 31,
		2013			2012
Beginning balance		$	6,651		$	4,393
Change in fair value			26,875			2,258
Exercises			(678	)		—
Repurchases			(83	)		—
Conversions			(2,012	)		—


Ending balance		$	30,753		$	6,651





The money market funds noted above are included in cash and cash equivalents in the accompanying 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 years ended December 31, 2013 or 2012 except for the transfer out of the warrants to purchase redeemable convertible preferred stock as described below.

During the year 2013, as a result of the closing of the IPO, the warrants to purchase preferred stock were converted to warrants to purchase common stock. The resulting warrants to purchase common stock meet the criteria to be classified as permanent equity and are no longer required to be measured at fair value at each reporting period.

The fair value of the warrants to purchase preferred stock that were classified as liabilities was estimated using the Black-Scholes option pricing model at the date of issuance and on each re-measurement date. This method of valuation involves using inputs such as the fair value of the Company's various classes of preferred stock, stock price volatility, the contractual term of the warrants, risk free interest rates, and dividend yields. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement. See Note 6 for further discussions of the accounting for the warrants, as well as for a summary of the significant inputs and assumptions used to determine the fair value of the warrants.

The fair value of warrants to purchase common stock that are classified as liabilities was estimated using a Monte Carlo model up until September 30, 2013. This method of valuation involves using inputs such as the fair value of a share of common stock, stock price volatility, and the contractual term of the warrants. Due to the warrants being deeply in the money as of December 31, 2013, the valuation method to estimate the warrants to purchase common stock was changed to the Black-Scholes option pricing model. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement.

The Company measures eligible assets and liabilities at fair value, with changes in value recognized in earnings. Fair value treatment may be elected either upon initial recognition of an eligible asset or liability or, for an existing asset or liability, if an event triggers a new basis of accounting. The Company did not elect to remeasure any of its existing financial assets or liabilities, and did not elect the fair value option for any financial assets and liabilities transacted in the years ended December 31, 2013 or 2012.

Property and Equipment

Property and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:


Asset		Estimated Useful Life
Computer equipment and software		3 years
Office and laboratory equipment		3 years
Leasehold improvements		Shorter of the useful life or remaining lease term

The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No impairment losses have been recorded during the years ended December 31, 2013, 2012 or 2011.

Revenue Recognition

The company has primarily generated revenue through collaboration, license and research arrangements with collaboration partners for the development and commercialization of protein therapeutics.

The Company recognizes revenue in accordance with FASB ASC Topic 605, Revenue Recognition. Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met: (1) persuasive evidence of an arrangement exists; (2) delivery has occurred or services have been rendered; (3) the fee is fixed or determinable; and (4) collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company's consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

Multiple Element Revenue Arrangements

The Company enters into collaboration agreements from time to time, which are more fully described in Note 9. The arrangements generally contain multiple elements or deliverables, which may include (1) licenses, or options to obtain licenses, to the Company's technology, (2) research and development activities performed for the collaboration partner, (3) participation on Joint Development Committees, and (4) manufacturing clinical or preclinical material. Payments pursuant to these arrangements typically include non-refundable, up-front payments, milestone payments upon achieving significant development events, research and development reimbursements, sales milestones, and royalties on future product sales.

Effective January 1, 2011, the Company adopted ASU No. 2009-13, Multiple-Deliverable Revenue Arrangements (ASU 2009-13), which amends Topic 605-25, Revenue Recognition—Multiple Element Arrangements (ASC 605-25). The Company applies this guidance to new arrangements as well as existing agreements that are significantly modified after January 1, 2011. For agreements that are significantly modified, the Company determines the estimated selling price for the remaining undelivered elements as of the date of the material modification and allocates arrangement consideration based upon the estimated selling price to the undelivered elements.

The application of the multiple element guidance requires subjective determinations, and requires management to make judgments about the individual deliverables, and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that: (1) the delivered item(s) has value to the customer on a stand-alone basis and (2) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially in the control of the Company. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have stand-alone value, based on the consideration of the relevant facts and circumstances for each arrangement, such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use the other deliverable(s) for their intended purpose without the receipt of the remaining element(s), whether the value of the deliverable is dependent on the undelivered item(s) and whether there are other vendors that can provide the undelivered element(s). Arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria, as described above, are applied to each of the separate units of accounting in determining the appropriate period or pattern of recognition.

The Company determines the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (VSOE) of selling price, if available, third-party evidence (TPE) of selling price if VSOE is not available, or management's best estimate of selling price (BESP) if neither VSOE nor TPE is available. Subsequent to the adoption of ASU 2009-13, the Company typically uses BESP to estimate the selling price of the deliverables. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

The Company typically receives up-front, non-refundable payments when licensing its intellectual property in conjunction with a collaboration agreement. When management believes the license to its intellectual property does not have stand-alone value from the other deliverables to be provided in the arrangement, the Company generally recognizes revenue attributed to the license on a straight-line basis over the contractual or estimated performance period, which is typically the term of the Company's research and development or manufacturing obligations. The Company continually evaluates these periods, and will adjust the period of revenue recognition if circumstances change. When management believes the license to its intellectual property has stand-alone value, the Company generally recognizes revenue attributed to the license upon delivery.

Research and development funding is recognized as revenue in the period that the related services are performed. When the Company acts as the principal under its collaboration agreements, it records payments received for the reimbursement of research and development costs as cost-sharing revenue in the consolidated statements of operations and comprehensive income (loss). To the extent that the Company reimburses the collaborator for costs incurred, the Company records these costs as a reduction of cost-sharing revenue.

The Company's agreements may contain options which provide the collaboration partner the right to obtain additional licenses. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors considered in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. For arrangements under which an option is considered substantive, the Company does not consider the item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration, assuming the option is not priced at a significant and incremental discount. Conversely, for arrangements under which an option is not considered substantive or if an option is priced at a significant and incremental discount, the Company would consider the item underlying the option to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration.

Effective January 1, 2011, the Company adopted ASU No. 2010-17, Revenue Recognition—Milestone Method (ASU 2010-17). At the inception of each arrangement that includes milestone payments, the Company evaluates, with respect to each milestone, whether the milestone is substantive and at-risk. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (1) the entity's performance to achieve the milestone, or (2) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting at least in part from the entity's performance to achieve the milestone, (b) the consideration relates solely to past performance, and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, regulatory, commercial, and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone, and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. On the milestone achievement date, assuming all other revenue recognition criteria are met and the milestone is deemed substantive and at-risk, the Company recognizes the payment as license and milestone revenue. For milestones that are not deemed substantive and at-risk, where payment is reasonably assured, the Company recognizes the milestone payment over the remaining service period.

Sales and commercial milestones and royalties will be recognized when and if earned, provided collectability is reasonably assured.

Contract Manufacturing Revenue

Contract manufacturing revenue is recognized upon delivery of the product in accordance with the terms of the contract, which specifies when transfer of title and risk of loss occurs.

Research and Development Expenses

Research and development costs are charged to expense as costs are incurred in performing research and development activities. Research and development costs include all direct costs, including salaries, stock compensation and benefits for research and development personnel, outside consultants, costs of clinical trials, sponsored research, clinical trials insurance, other outside costs, depreciation and facility costs related to the development of drug candidates. The Company records up-front, non-refundable payments made to outside vendors, or other payments made in advance of services performed or goods being delivered, as prepaid expenses, which are expensed as services are performed or the goods are delivered.

Certain research and development projects are, or have been, partially funded by collaboration agreements, and the expenses related to these activities are included in research and development costs. The Company records the related reimbursement of research and development costs under these agreements as revenue, as more fully described above and in Note 9.

Stock-Based Compensation

At December 31, 2013, the Company had two stock-based compensation plans, which are more fully described in Note 10. The Company accounts for stock-based compensation in accordance with the provisions of ASC Topic 718, Compensation—Stock Compensation (ASC 718), which requires the recognition of expense related to the fair value of stock-based compensation awards in the statements of operations and comprehensive income (loss).

For stock options issued to employees and members of the Board for their services on the Board, the Company estimates the grant date fair value of each option using the Black-Scholes option-pricing model. The use of the Black-Scholes option-pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the common stock consistent with the expected life of the option, risk-free interest rates and expected dividend yields of the common stock. For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense, net of estimated forfeitures, equal to the grant date fair value of stock options on a straight-line basis over the requisite service period, which is generally the vesting term. For awards subject to both performance and service-based vesting conditions, the Company recognizes stock-based compensation expense using an accelerated recognition method when it is probable that the performance condition will be achieved. Forfeitures are required to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.

Share-based payments issued to non-employees are recorded at their fair values, and are periodically revalued as the equity instruments vest and are recognized as expense over the related service period in accordance with the provisions of ASC 718 and ASC Topic 505, Equity. For stock-based awards granted to non-employees, the Company recognizes stock-based compensation expense using an accelerated recognition method.

See Note 10 for a discussion of the assumptions used by the Company in determining the grant date fair value of options granted under the Black-Scholes option pricing model, as well as a summary of the stock option activity under the Company's stock-based compensation plan for the year ended December 31, 2013.

Income Taxes

Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (ASC 740), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2013 and 2012, the Company does not have any significant uncertain tax positions.

Net Income (Loss) Per Share

Net income (loss) per share information is determined using the two-class method, which includes the weighted-average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). The Company's redeemable convertible preferred stock are participating securities as defined by ASC 260-10, Earnings Per Share.

Under the two-class method, basic net income (loss) per share applicable to common stockholders is computed by dividing the net income (loss) applicable to common stockholders by the weighted-average number of common shares outstanding for the reporting period. Diluted net income (loss) per share is computed using the more dilutive of (1) the two-class method or (2) the if-converted method. The Company allocates net income first to preferred stockholders based on dividend rights under the Company's articles of incorporation and then to preferred and common stockholders based on ownership interests. Net losses are not allocated to preferred stockholders as they do not have an obligation to share in the Company's net losses.

Diluted net income (loss) per share gives effect to all potentially dilutive securities, including redeemable convertible preferred stock, and shares issuable upon the exercise of outstanding warrants and stock options, using the treasury stock method. For the years ended December 31, 2013 and 2012, the Company has excluded the effects of all potentially dilutive shares, which include redeemable convertible preferred stock, warrants for redeemable convertible preferred stock, warrants for common stock and outstanding common stock options, from the weighted-average number of common shares outstanding as their inclusion in the computation for these two years would be anti-dilutive due to net losses.

The following common stock equivalents were excluded from the calculation of diluted net income (loss) per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands):


		Year Ended December 31,
		2013			2012		2011
Outstanding stock options			3,709			3,730		—
Common stock warrants			908			884		874
Preferred stock			13,218			18,166		—
Preferred stock warrants			114			248		—


			17,949			23,028		874





Comprehensive Income (Loss)

Comprehensive income (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 income (loss) consists of net income (loss) and other comprehensive income (loss), which includes certain changes in equity that are excluded from net income (loss). Comprehensive income (loss) has been disclosed in the accompanying consolidated statements of operations and comprehensive (loss) income and equals the Company's net income (loss) for all periods presented.

Subsequent Events

The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. See Note 15.

Application of New or Revised Accounting Standards

On April 5, 2012, the Jump-Start Our Business Startups Act (the JOBS Act) was signed into law. The JOBS Act contains provisions that, among other things, reduce certain reporting requirements for an "emerging growth company." As an emerging growth company the Company has elected to not take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards, and as a result, will comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non-emerging growth companies.

Recently Adopted Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

Property_and_Equipment_Net

Property and Equipment, Net	12 Months Ended
Property and Equipment, Net	Dec. 31, 2013
Property and Equipment, Net	'
Property and Equipment, Net	'

	3. Property and Equipment, Net

	Property and equipment, net, consists of the following (in thousands):


		December 31,
		2013			2012
	Computer equipment and software	$	921		$	919
	Office equipment		205			179
	Laboratory equipment		12,334			11,815
	Leasehold improvements		9,930			10,088
	Construction in progress		—			8


	Total property and equipment		23,390			23,009
	Accumulated depreciation and amortization		(19,685	)		(18,950	)


	Property and equipment, net	$	3,705		$	4,059





	Depreciation and amortization expense was $0.9 million, $1.3 million and $3.1 million for the years ending December 31, 2013, 2012 and 2011, respectively.

Restricted_Cash

Restricted Cash	12 Months Ended
Restricted Cash	Dec. 31, 2013
Restricted Cash	'
Restricted Cash	'

	4. Restricted Cash

	As of December 31, 2013 and 2012, the Company maintained letters of credit totaling $0.9 million held in the form of a money market account as collateral for the Company's facility lease obligations and its credit cards.

Accrued_Expenses

Accrued Expenses	12 Months Ended
Accrued Expenses	Dec. 31, 2013
Accrued Expenses	'
Accrued Expenses	'

	5. Accrued Expenses

	Accrued expenses consist of the following (in thousands):


		December 31,
		2013		2012
	Collaboration expense	$	—	$	1,000
	Research and development related		1,330		1,282
	Employee compensation		2,930		2,448
	Professional services		1,017		607
	Other		1,650		816


		$	6,927	$	6,153

Warrants

Warrants	12 Months Ended
Warrants	Dec. 31, 2013
Warrants	'
Warrants	'

	6. 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
			Warrants as of									Balance Sheet
									Weighted-			Classification
									Average
			December 31,						Exercise
			December 31,						Price Per			December 31,	December 31,
			2013			2012			Share		Expiration	2013	2012
	Warrant to purchase Series A Preferred Stock			—			107		$	4	February 28, 2013	N/A(1)	Liability
	Warrants to purchase Series B Preferred Stock			—			32			7.4	December 21, 2013	N/A(2)(4)	Liability
	Warrants to purchase Series C-1 Preferred Stock			—			46			10.92	June 25, 2019	N/A(2)	Liability
	Warrants to purchase Series D-1 Preferred Stock			—			64			12.56	March 18, 2020	N/A(2)	Liability
	Warrants to purchase Common Stock			46			—			10.92	June 25, 2019	Equity(2)	N/A
	Warrants to purchase Common Stock			64			—			12.56	March 18, 2020	Equity(2)	N/A
	Warrants to purchase Common stock			857			872			5.88	June 10, 2020 - July 9, 2020	Liability	Liability
	Warrants to purchase Common stock			13			13			4.00 - 7.40	March 31, 2015 - December 31, 2017	Equity(3)	Equity


	All warrants			980			1,134		$	6.53






	-1
	On February 6, 2013, the warrant holder exercised a warrant to purchase 107 shares of Series A Preferred Stock on a net basis, resulting in the issuance of 47 shares of Series A Preferred Stock.

	-2
	Warrants to purchase Series B Preferred Stock, Series C-1 Preferred Stock, and Series D-1 Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

	-3
	Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to December 31, 2012. See discussion below for further details.

	-4
	On December 19 and 20, 2013, the warrants holders exercised warrants to purchase 32 shares of Common Stock on a net basis, resulting in the issuance of 24 shares of Common Stock.

	In connection with various financing transactions that were consummated in periods prior to December 31, 2012, the Company issued warrants for the purchase of up to 106,500 shares of the Company's Series A redeemable convertible preferred stock (Series A Preferred Stock), 31,891 shares of the Company's Series B redeemable convertible preferred stock (Series B Preferred Stock), 45,786 shares of the Company's Series C-1 redeemable convertible preferred stock (Series C-1 Preferred Stock), and 63,693 shares of the Company's Series D-1 redeemable convertible preferred stock (Series D-1 Preferred Stock). Each warrant was immediately exercisable. The warrants to purchase Series A and Series B Preferred Stock expire seven years from the original date of issuance, while the warrants to purchase Series C-1 and Series D-1 Preferred Stock expire ten years from the original date of issuance. The warrants to purchase shares of the Company's preferred stock have an exercise price equal to the original issuance price of the underlying instrument. Each warrant is exercisable on either a physical settlement or net share settlement basis and the redemption provisions are outside the control of the Company. In connection with the closing of the Company's IPO on September 24, 2013, the outstanding warrants to purchase Series B Preferred Stock, Series C-1 Preferred Stock, and Series D-1 Preferred Stock were converted into warrants to purchase common stock The exercise prices for each of these warrants remained unchanged.

	The Company follows the provisions of ASC Topic 480, Issuer's Accounting for Freestanding Warrants and Other Similar Instruments on Shares that Are Redeemable, which requires that warrants to purchase redeemable preferred stock be classified as liabilities. In addition, the value of the warrants is remeasured to the then-current fair value at each reporting date. Changes in fair value are recorded to other income (expense), net. For the years ended December 31, 2013, 2012 and 2011, the Company remeasured the fair value of all of its outstanding warrants to purchase shares of the Company's preferred stock up until the conversion of such warrants on September 24, 2013, using current assumptions, resulting in an increase in fair value of $1.3 million, $0.4 million, and $0.1 million, respectively, which was recorded in other expense net in the accompanying consolidated statements of operations and comprehensive (loss) income. As a result of the closing of the IPO and the resulting conversion of the warrants to purchase preferred shares into warrants to purchase common stock, the fair value of the warrant liability at September 24, 2013 was reclassified to permanent equity and therefore, is no longer subject to remeasurement.

	In December 2012, the Company modified the warrant to purchase 106,500 shares of Series A Preferred Stock and extended the expiration date from December 21, 2012 to February 28, 2013. During the year ended December 31, 2013, the holder of the warrant exercised the warrant on a net basis, resulting in the issuance of 46,668 shares of Series A Preferred Stock. Upon exercise, the Company re-measured the fair value of the warrant and recorded the resulting increase in fair value of $0.1 million as other expense in the accompanying statement of operations and comprehensive income for the year ended December 31, 2013.

	In connection with the Series E redeemable convertible preferred stock (Series E Preferred Stock) financing transactions that took place in June 2010 and July 2010, the Company issued warrants to purchase up to 871,580 shares of common stock. Each warrant was immediately exercisable and expires ten years from the original date of issuance. The warrants to purchase shares of the Company's common stock have an exercise price equal to the estimated fair value of the underlying instrument as of the initial date such warrants were issued. Each warrant is exercisable on either a physical settlement or net share settlement basis from the date of issuance. The warrant agreement contains a provision requiring an adjustment to the number of shares in the event the Company issues common stock, or securities convertible into or exercisable for common stock, at a price per share lower than the warrant exercise price. The Company concluded the anti-dilution feature required the warrants to be classified as liabilities under ASC Topic 815, Derivatives and Hedging—Contracts in Entity's Own Equity (ASC 815). The warrants are measured at fair value, with changes in fair value recognized as a gain or loss to other income (expense) in the consolidated statements of operations and comprehensive income (loss) for each reporting period thereafter. The fair value of the common stock warrants were recorded as a discount to the preferred stock issued of $3.0 million, and the preferred stock were being accreted to the redemption value. At the end of each reporting period, the Company remeasured the fair value of the outstanding warrants, using current assumptions, resulting in an increase in fair value of $26.9 million, $1.9 million and $0.3 million, respectively, which was recorded in other expense in the accompanying consolidated statements of operations and comprehensive (loss) income for the years ended December 31, 2013, 2012 and 2011. The Company will continue to re-measure the fair value of the liability associated with the warrants to purchase common stock at the end of each reporting period until the earlier of the exercise or the expiration of the applicable warrants. On March 31, 2013, the Company retired 13,994 warrants to purchase common stock as a consequence of a repurchase of shares from an investor. All remaining outstanding warrants were fully vested and exercisable as of December 31, 2013.

	In connection with various financing transactions that were consummated in periods prior to December 31, 2012, the Company issued warrants to purchase up to 12,634 shares of common stock. The awards of warrants to purchase shares of common stock are accounted for as equity instruments. The warrants are exercisable at any time through their respective expiration dates. The fair value at issuance was calculated using the Black-Scholes option-pricing model, and was charged to interest expense during the periods the related debt was outstanding.

	The Company issued warrants to purchase up to 41,388 shares of common stock in periods prior to December 31, 2012 in exchange for consulting services provided by a third party pursuant to stand-alone award agreements that are independent of an equity incentive plan. The warrants vested upon achievement of four milestones and were outstanding for approximately seven years from the date of issuance. During the year ended December 31, 2011, the holder exercised 41,388 warrants to purchase common stock on a net basis resulting in the issuance of 37,249 shares of common stock. There were no exercises, cancellations, or expirations of warrants during the years ended December 31, 2013 or 2012.

	Fair Value

	The fair value of the warrants to purchase preferred stock on the date of issuance and on each re-measurement date was estimated using the Black-Scholes option pricing model. This method of valuation involves using inputs such as the fair value of the Company's various classes of preferred stock and common stock, stock price volatility, contractual term of the warrants, risk free interest rates, and dividend yields. The fair value of the warrants to purchase common stock on the date of issuance and on each re-measurement date for those warrants to purchase common stock classified as liabilities was estimated using the Monte Carlo simulation framework, up until September 30, 2013, which incorporated three future financing events over the remaining life of the warrants to purchase common stock. Due to the warrants being deeply in the money as of December 31, 2013, the valuation method to estimate the warrants to purchase common stock was changed to the Black-Scholes option pricing model. Due to the nature of these inputs and the valuation techniques utilized, the valuation of the warrants to purchase preferred stock and common stock are considered a Level 3 measurement (Note 2).

	The fair value of each warrant to purchase shares of the Company's Series A Preferred Stock was estimated using the Black-Scholes option pricing model with the following assumptions:


			Year Ended December 31,
			2013(2)			2012(1)			2011
Fair value of underlying instrument			n/a		$	9.24		$	6.76
Expected volatility			n/a			69.1	%		66	%
Expected term (in years)			n/a			0.16			1.16
Risk-free interest rate			n/a			0.04	%		0.12	%
Expected dividend yield			n/a			—	%		—	%


-1
During December 2012, the expiration date of the warrant to purchase Series A Preferred Stock was extended from December 21, 2012 to February 28, 2013.

-2
The warrant to purchase Series A Preferred Stock was exercised during the three months ended March 31, 2013.

The fair value of each warrant to purchase shares of the Company's Series B Preferred Stock was estimated using the Black-Scholes option pricing model with the following assumptions:


		Year Ended December 31,
		2013(1)			2012			2011
Fair value of underlying instrument		$	20.03		$	9.96		$	7.56
Expected volatility			71.2	%		69.1	%		66	%
Expected term (in years)			0.24			0.97			1.98
Risk-free interest rate			0.04	%		0.16	%		0.25	%
Expected dividend yield			—	%		—	%		—	%


-1
Warrants to purchase Series B Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

The fair value of each warrant to purchase shares of the Company's Series C-1 Preferred Stock was estimated using the Black-Scholes option pricing model with the following assumptions:


		Year Ended December 31,
		2013(1)			2012			2011
Fair value of underlying instrument		$	20.03		$	11.04		$	8.84
Expected volatility			71.2	%		69.1	%		66	%
Expected term (in years)			5.73			6.46			7.46
Risk-free interest rate			1.75	%		0.95	%		1.35	%
Expected dividend yield			—	%		—	%		—	%


-1
Warrants to purchase Series C-1 Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

The fair value of each warrant to purchase shares of the Company's Series D-1 Preferred Stock was estimated using the Black-Scholes option pricing model with the following assumptions:


		Year Ended December 31,
		2013(1)			2012			2011
Fair value of underlying instrument		$	20.03		$	10.52		$	8.84
Expected volatility			71.2	%		69.1	%		66	%
Expected term (in years)			6.48			7.22			8.22
Risk-free interest rate			1.75	%		1.18	%		1.62	%
Expected dividend yield			—	%		—	%		—	%


-1
Warrants to purchase Series D-1 Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

Fair Value of Underlying Instrument

The Company estimated the fair value of its shares of Series A Preferred Stock, Series B-1 Preferred Stock, Series C-1 Preferred Stock and Series D-1 Preferred Stock as of December 31, 2012 and 2011, and through the exercise or conversion of its Preferred stock warrants during 2013, using the PWERM (probability-weighted expected return method).

Expected Volatility

The Company estimated the expected volatility based on actual historical volatility of the stock price of similar companies with publicly-traded equity securities. The Company calculated the historical volatility of the selected companies by using daily closing prices over a period of the expected term of the associated award. The companies were selected based on their enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the associated award. A decrease in the selected volatility would decrease the fair value of the underlying instrument.

Expected Term

The Company based the expected term on the actual remaining contractual term of each respective warrant. A decrease in the expected term would decrease the fair value of the underlying instrument.

Risk-Free Interest Rate

The Company estimated the risk-free interest rate in reference to the yield on U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. A decrease in the selected risk-free rate would decrease the fair value of the underlying instrument.

Expected Dividend Yield

The Company estimated the expected dividend yield based on consideration of its historical dividend experience and future dividend expectations. The Company has not historically declared or paid dividends to stockholders. Moreover, it does not intend to pay dividends in the future, but instead expects to retain any earnings to invest in the continued growth of the business. Accordingly, the Company assumed an expected dividend yield of 0.0%.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2013
Commitments and Contingencies	'
Commitments and Contingencies	'

	7. Commitments and Contingencies

	Operating Leases

	The Company leases its facilities under non-cancelable operating leases that expire at various dates through May 2018. All of the Company's leases contain escalating rent clauses, which require higher rent payments in future years. The Company expenses rent on a straight-line basis over the term of the lease, including any rent-free periods. In addition, the Company received certain leasehold improvement incentives, and recorded these incentives as deferred rent, which is amortized as a reduction of rent expense over the life of the lease. Rent expense of approximately $3.5 million, $3.5 million and $3.6 million were incurred during the years ended December 31, 2013, 2012 and 2011, respectively.

	Future annual minimum lease payments as of December 31, 2013, are as follows (in thousands):


	2014	$	4,522
	2015		4,106
	2016		3,938
	2017		3,938
	2018		2,953


	Total	$	19,457





	In February 2011, the Company entered into a sublease agreement for a portion of one of its facility leases. The tenant will pay rent on the lease from February 28, 2011 until May 30, 2015. On December 31, 2013, the sublease agreement was expanded to include the remaining space of that facility. The expansion portion will be leased for one year, through December 31, 2014. The Company will continue to utilize the expansion portion of the leased property, once the sublease expires.

	Future annual minimum sublease proceeds as of December 31, 2013, are as follows (in thousands):


	2014	$	832
	2015		252


	Total	$	1,084





	Legal Proceedings

	On October 18, 2012, the Salk Institute for Biological Studies (Salk) filed a complaint in the Massachusetts Superior Court for Suffolk County, alleging that the Company breached one of the Company's two licensing agreements with Salk. The licensing agreement in dispute provides the Company with a license with respect to certain of Salk's U.S. patents related to the ActRIIB activin receptor proteins. Salk contends that, under the licensing agreement, the Company owed Salk a greater share of the upfront payment that it received under its now-terminated agreement with Shire AG regarding ACE-031 and a share of the upfront payment and development milestone payments that the Company has received under its ongoing collaboration agreement with Celgene regarding ACE-536. Salk is seeking a total of approximately $10.5 million plus interest in payment and a 15% share of future development milestone payments received under the agreement with Celgene regarding ACE-536. The Company contends that no additional amounts are due to Salk and that it has complied with all of its payment obligations under the applicable Salk license agreement.

	The Company moved to dismiss the complaint on December 3, 2012. The Court denied the Company's motion on February 28, 2013. On March 14, 2013, Acceleron answered the complaint and asserted patent invalidity counterclaims. On the basis of those counterclaims, Acceleron removed the action on March 28, 2013 to the United States District Court for the District of Massachusetts. The parties have since reached an agreement on a stipulation as to certain patent issues raised in the action, and Acceleron has dismissed its counterclaims. The Court held an initial scheduling conference on May 30, 2013, and the fact discovery is now closed. The case is currently scheduled for trial in September 2014. The Company intends to defend its position vigorously.

	The Company evaluated the suit under ASC Topic 450, Contingencies, as a loss contingency. The estimated loss from a loss contingency shall be accrued if information available before the financial statements are issued indicates that it is probable a liability had been incurred at the date of the financial statements, and the amount of loss can be reasonably estimated. Because the Company believes that the potential for an unfavorable outcome is not probable, it has not established a reserve with respect to the dispute as of December 31, 2013 or 2012.

	The Company's estimates can be affected by various factors. As of December 31, 2013 and 2012, management has determined a loss is reasonably possible. Although the Company believes it would successfully defend the lawsuit, the Company has in the past participated in settlement discussions with Salk. Accordingly, the Company has estimated the range of possible losses as of December 31, 2013 and 2012 to be between $0 and $10.5 million plus interest.

	Other

	The Company is also party to various agreements, principally relating to licensed technology, that require future payments relating to milestones not met at December 31, 2013 and 2012, or royalties on future sales of specified products. No milestone or royalty payments under these agreements are expected to be payable in the immediate future. See Note 9 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.

Redeemable_Convertible_Preferr

Redeemable Convertible Preferred Stock and Stockholders' Equity	12 Months Ended
	Dec. 31, 2013
Redeemable Convertible Preferred Stock and Stockholders' Equity	'
Redeemable Convertible Preferred Stock and Stockholders' Equity	'

	8. Redeemable Convertible Preferred Stock and Stockholders' Equity

	As of December 31, 2012, the authorized capital stock of the Company was 104,013,161 shares of common stock, par value $0.001 per share and 74,077,227 shares of preferred stock, par value $0.001 per share, of which: (1) 26,069,980 shares have been designated as Series A Preferred Stock, (2) 16,944,378 shares have been designated as Series B Preferred Stock, (3) 11,923,077 shares have been designated as Series C redeemable convertible (Series C Preferred Stock), (4) 2,014,652 shares have been designated as Series C-1 Preferred Stock, (5) 955,414 shares have been designated as Series D redeemable convertible preferred stock (Series D Preferred Stock), (6) 2,802,548 shares have been designated as Series D-1 Preferred Stock, (7) 3,662,422 shares have been designated as Series E Preferred Stock, and (8) 9,704,756 shares have been designated as Series F redeemable convertible preferred stock (Series F Preferred Stock, and all collectively the Preferred Stock).

	On September 4, 2013, the Board and the stockholders of the Company approved a one-for-four reverse stock split of the Company's outstanding common stock, which was effected on September 5, 2013. Stockholders entitled to fractional shares as a result of the reverse stock split received a cash payment in lieu of receiving fractional shares. The Company's historical share and per share information have been retroactively adjusted to give effect to this reverse stock split. Shares of common stock underlying outstanding stock options and other equity instruments were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the agreements governing such securities.

	On September 4, 2013, the Board also approved for filing immediately following the effectiveness of the Company's registration statement in connection with its IPO, the Restated Certificate of Incorporation to increase the authorized number of shares of common stock from 104,013,161 to 175,000,000, to authorize 25,000,000 shares of undesignated preferred stock, par value $0.001 per share, and to eliminate all references to the previously designated Series Preferred Stock. This Restated Certificate of Incorporation was approved by the stockholders on September 4, 2013.

	On September 24, 2013 the Company completed its initial public offering (IPO) whereby the Company sold 6,417,000 shares of common stock (including 837,000 shares of common stock sold by the Company pursuant to the full exercise of an overallotment option by the underwriters in connection with the offering) at a price of $15.00 per share. The shares began trading on The Nasdaq Global Market on September 19, 2013. The aggregate net proceeds received by the Company from the offering were $86.8 million, net of underwriting discounts and commissions and estimated offering expenses payable by the Company. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 18,516,993 shares of common stock and warrants exercisable for convertible preferred stock were automatically converted into warrants exercisable for 141,370 shares of common stock, resulting in the reclassification of the related convertible preferred stock warrant liability of $2.0 million to additional paid-in capital.

	On September 24, 2013 the Company also completed the sale of a private placement of 666,667 shares of common stock to Celgene Corporation at the IPO price of $15.00 per share concurrent with and at the same offer price as the IPO. The aggregate net proceeds received by the Company from the concurrent private placement were $10.0 million.

	Redeemable Convertible Preferred Stock

	Prior to the closing of the initial public offering, at which time all shares of Preferred Stock were converted into shares of common stock, the Company's Preferred Stock consisted of the following (in thousands, except share and per share data):


		December 31,
		2012
	Series A Preferred Stock, $0.001 par value: 26,069,980 shares authorized, 6,410,976 shares issued and outstanding at December 31, 2012, at redemption value	$	66,665
	Series B Preferred Stock, $0.001 par value: 16,944,378 shares authorized, 4,204,185 shares issued and outstanding at December 31, 2012, at redemption value		67,044
	Series C Preferred Stock, $0.001 par value: 11,923,077 shares authorized, 2,978,062 shares issued, and outstanding at December 31, 2012, at redemption value		59,909
	Series C-1 Preferred Stock, $0.001 par value: 2,014,652 shares authorized, 457,875 issued, and outstanding at December 31, 2012, at redemption value		9,387
	Series D Preferred Stock, $0.001 par value: 955,414 shares authorized, 234,940 shares issued, and outstanding at December 31, 2012, at redemption value		4,325
	Series D-1 Preferred Stock, $0.001 par value: 2,802,548 shares authorized, 636,942 issued and outstanding at December 31, 2012, at redemption value		11,864
	Series E Preferred Stock, $0.001 par value: 3,662,422 shares authorized, 816,060 shares issued and outstanding at December 31, 2012, at redemption value		13,393
	Series F Preferred Stock, $0.001 par value: 9,704,756 shares authorized, 2,426,171 issued and outstanding at December 31, 2012, at redemption value		36,023


	Total redeemable convertible preferred stock	$	268,610





	The holders of the Company's Preferred Stock had certain voting, dividend, and redemption rights, as well as liquidation preferences and conversion privileges. All rights, preferences, and privileges associated with the Preferred Stock were terminated at the time of the Company's IPO in conjunction with the conversion of all outstanding shares of Preferred Stock into shares of common stock.

	Common Stock

	The holders of shares of common stock are entitled to one vote for each share of common stock held at all meetings of stockholders and written actions in lieu of meetings.

	The holders of shares of common stock are entitled to receive dividends, if and when declared by the Board. No dividends have been declared or paid by the Company through December 31, 2013.

	Common Stock Reserved for Future Issuance

	At December 31, 2013, the Company has reserved for future issuance the following number of shares of common stock (in thousands):


		December 31,
		2013
	Outstanding stock options to purchase common stock		3,942
	Shares available for future issuance under stock option plan		956
	Warrants to purchase common stock		980
	Additional shares reserved for unissued, but designated, Preferred Stock		25,000


	Total shares of authorized common stock reserved for future issuance		30,878

Significant_Agreements

Significant Agreements	12 Months Ended
Significant Agreements	Dec. 31, 2013
Significant Agreements	'
Significant Agreements	'

	9. Significant Agreements

	Celgene

	Overview

	On February 20, 2008 the Company entered into a collaboration, license, and option agreement (the Sotatercept Agreement) with Celgene Corporation (Celgene) relating to sotatercept. On August 2, 2011, the Company entered into a second collaboration, license and option agreement with Celgene for ACE-536 (the ACE-536 Agreement), and also amended certain terms of the Sotatercept Agreement. These agreements provide Celgene exclusive licenses for Sotatercept and ACE-536 in all indications, as well as exclusive rights to obtain a license to certain future compounds. Celgene is a global biopharmaceutical company primarily engaged in the discovery, development and commercialization of innovative therapies designed to treat cancer and immune-inflammatory related diseases.

	Sotatercept Agreement

	Under the terms of the Sotatercept Agreement, the Company and Celgene collaborate worldwide for the joint development and commercialization of sotatercept. The Company also granted Celgene an option to license three discovery stage compounds. Under the terms of the agreement, the Company and Celgene will jointly develop, manufacture and commercialize sotatercept. Celgene paid $45.0 million of nonrefundable, upfront license and option payments to the Company upon the closing of the Sotatercept Agreement.

	The Company retained responsibility for research and development through the end of Phase 2a clinical trials, as well as manufacturing the clinical supplies for these trials. These activities were substantially completed in 2011. Celgene is conducting the ongoing Phase 2 trials for myelodysplastic syndromes (MDS), chronic kidney disease, and b-thalassemia and will be responsible for any Phase 3 clinical trials, as well as additional Phase 2 clinical trials, and will be responsible for overseeing the manufacture of Phase 3 and commercial supplies by third-party contract manufacturing organizations. Under the agreement, the Company was eligible to receive clinical milestones of up to $88.0 million, regulatory milestones of up to $272.0 million, and commercial milestones of up to $150.0 million for sotatercept. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon the acceptance of the marketing application and upon the approval to market a product candidate by the Food and Drug Administration (FDA) or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by Celgene in countries outside of North America. In addition, to the extent sotatercept is commercialized, the Company would be entitled to receive tiered royalty payments in the low-to-mid twenty percent range of net sales from sales generated from all geographies. Royalty payments are subject to certain reductions, including for entry of a generic product onto the market.

	Additionally, for three named discovery-stage option programs the Company was eligible to receive option fees of up to $30.0 million, clinical milestones of up to $53.3 million, regulatory milestones of up to $204.0 million, and commercial milestones of up to $150.0 million for each option program. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon the acceptance of the marketing application and upon the approval to market a product candidate by the FDA or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by Celgene in countries outside of North America. Option fee payments are triggered upon license of any of the option programs by Celgene. In addition, to the extent an option compound is commercialized, the Company would be entitled to receive tiered royalty payments in the low-to-mid twenty percent range of net sales from sales generated from all geographies. Royalty payments are subject to certain reductions, including for entry of a generic product onto the market. None of the three discovery stage programs has advanced to the stage to achieve payment of a milestone.

	In connection with entering into the Sotatercept Agreement, Celgene purchased 457,875 shares of Series C-1 Preferred Stock at the aggregate purchase price of $5.0 million. The Series C-1 Preferred Stock was purchased at an amount that was deemed to represent fair value at the time of purchase.

	Commensurate with the execution of the ACE-536 Agreement described below, the Company and Celgene agreed to modify the terms of the Sotatercept Agreement. The modified terms included: (1) a change to the responsibility for development costs to align with the ACE-536 Agreement, with Celgene responsible for more than half of the worldwide costs through December 31, 2012, and 100% of the development costs thereafter, (2) future contingent development milestones for sotatercept were amended to a two-category (oncology and non-oncology) structure with potential future clinical milestones of $27.0 million and regulatory milestones of $190.0 million from a four-category (various cancer indications) structure and, (3) future contingent development milestones for option compounds were amended to a two-category (oncology and non-oncology) structure with potential future clinical milestones of $25.5 million and regulatory milestones of $142.5 million from a four-category (various cancer indications) structure, and (4) an option to buy down tiered royalty payments on both Sotatercept and ACE-536 with a one-time $25.0 million payment on or prior to January 1, 2013. The potential commercial milestones remained unchanged. As of December 31, 2013, the Company has received $41.6 million in research and development funding and milestone payments for sotatercept under the original and modified agreements, of which $7.0 million is a clinical milestone payment earned in December 2013 that resulted from Celgene's start of a Phase 2b clinical trial in chronic kidney disease.

	The Sotatercept Agreement will expire on a country-by-country basis on the occurrence of both of the following: (1) the expiration of the royalty term with respect to all license products in such country, and (2) the exercise or forfeiture by Celgene of its option with regard to each option compound. The royalty term for each licensed product in each country outside North America is the period commencing with first commercial sale of the applicable licensed product in the applicable country and ending on the latest of expiration of specified patent coverage or a specified period of years. The royalty term for each licensed product in North America is the period commencing with the first commercial sale in North America and ending, on a licensed product and country-by-country basis on the date which commercialization of such licensed product has ceased. The term for each option compound runs for a specified period of years unless Celgene exercises its option, in which case the compound becomes a licensed product, or forfeits its option by failing to make certain payments following the achievement of certain milestones in early clinical development of the option compound.

	Celgene has the right to terminate the agreement with respect to one or more licensed targets or in its entirety, upon 180 days' notice (or 45 days' notice if the licensed product has failed to meet certain end point criteria with respect to clinical trials or other development activities). The agreement may also be terminated in its entirety by either Celgene or the Company in the event of a material breach by the other party or in the event of a bankruptcy filing of the other party. There are no cancellation, termination or refund provisions in this arrangement that contain material financial consequences to the Company.

	ACE-536 Agreement

	Under the terms of the ACE-536 Agreement, the Company and Celgene collaborate worldwide for the joint development and commercialization of ACE-536. The Company also granted Celgene an option for future products for which Acceleron files an Investigational New Drug application for the treatment of anemia. Celgene paid $25.0 million on the closing of the ACE-536 Agreement in August, 2011.

	Under the terms of the ACE-536 Agreement the Company retains responsibility for research and development through the end of Phase 1 and initial Phase 2 clinical trials, as well as manufacturing the clinical supplies for these studies. Celgene will conduct subsequent Phase 2 and Phase 3 clinical studies. Acceleron will manufacture ACE-536 for the Phase 1 and Phase 2 clinical trials and Celgene will be responsible for overseeing the manufacture of Phase 3 and commercial supplies by third party contract manufacturing organizations. The Company is eligible to receive clinical milestones of up to $32.5 million, regulatory milestones of up to $105.0 million and commercial milestones of up to $80.0 million for ACE-536. The Company will receive additional, lower development, regulatory, and commercial milestones for any additional products for the treatment of anemia on which Celgene exercises an option. Clinical milestone payments are triggered upon initiation of a defined phase of clinical research for a product candidate. Regulatory milestone payments are triggered upon the acceptance of the marketing application and upon approval to market a protein therapeutic candidate by the FDA or other global regulatory authorities. Commercial milestone payments are triggered when an approved pharmaceutical product reaches certain defined levels of net sales by Celgene in countries outside of North America. In addition, to the extent ACE-536 is commercialized, the Company would be entitled to receive tiered royalty payments in the low-to-mid twenty percent range of net sales from sales generated from all geographies. Royalty payments are subject to certain reductions, including for entry of a generic product onto the market.

	Through December 31, 2013, the Company has received $31.9 million in research and development funding and milestone payments for ACE-536. The next likely clinical milestone payment would be $15.0 million and result from the start of a Phase 3 study in MDS or b-thalassemia. The Company has not yet identified additional compounds for the treatment of anemia. Accordingly, there is no assurance that the Company will generate future value from additional programs.

	The ACE-536 Agreement will expire on a country-by-country basis on the occurrence of both of the following: (1) the expiration of the royalty term with respect to all license products in such country, and (2) the end of the option term. The royalty term for each licensed product in each country outside North America is the period commencing with first commercial sale of the applicable licensed product in the applicable country and ending on the latest of expiration of specified patent coverage or a specified period of years. The royalty term for each licensed product in North America is the period commencing with the first commercial sale in North America and ending, on a licensed product and country-by-country basis on the date which commercialization of such licensed product has ceased. The option term runs until the later of (1) the date on which no development or commercialization activities are ongoing or are expected to commence for any licensed products under the ACE-536 Agreement; (2) the date on which no development or commercialization activities are ongoing or are expected to commence for any licensed products under the Sotatercept Agreement and all option rights under the Sotatercept Agreement have been forfeited with respect to each option compound where Celgene has made a payment with respect to such compound; and (3) the royalty term for all licensed products under the ACE-536 Agreement and the Sotatercept Agreement has ended; provided that if at the time the option term would otherwise end any option compounds under the ACE-536 Agreement are in clinical development the option term shall continue until Celgene's rights to such compound are either exercised or forfeited.

	Celgene has the right to terminate the ACE-536 Agreement with respect to one or more licensed targets or in its entirety, upon 180 days' notice (or 45 days' notice if the licensed product has failed to meet certain end point criteria with respect to clinical trials or other development activities), provided that Celgene may not terminate the ACE-536 Agreement prior to the completion of the on-going ACE-536 b -thalassemia and ACE-536 MDS Phase 2 clinical trials, except under certain conditions. The agreement may also be terminated in its entirety by either Celgene or the Company in the event of a material breach by the other party or in the event of a bankruptcy filing of the other party. There are no cancellation, termination or refund provisions in this arrangement that contain material financial consequences to the Company.

	Both Agreements

	The Company and Celgene shared development costs under the Sotatercept and ACE-536 Agreements through December 31, 2012. As of January 1, 2013, Celgene is responsible for paying 100% of the worldwide development costs under both agreements. Celgene will be responsible for all commercialization costs worldwide. The Company has the right to co-promote sotatercept, ACE-536 and future products under both agreements in North America. Celgene's option to buy down royalty rates for sotatercept and ACE-536 expired unexercised and, therefore, the Company will receive tiered royalties in the low-to-mid twenty percent range on net sales of sotatercept and ACE-536. The royalty schedules for sotatercept and ACE-536 are the same.

	Accounting Analysis

	Prior to 2011, the Company accounted for the Sotatercept Agreement, as a multiple element arrangement under ASC 605-25 (prior to the amendments of ASU 2009-13). The Company identified the following deliverables under the arrangement; (1) the license to the ActRIIA compound, (2) right to license option program compounds, (3) participation in the joint development committee, (4) participation in the joint commercialization committee and (5) research and development activities. Under the provisions of ASC 605- 25, applicable to the arrangement, since the Company could not establish VSOE for the undelivered elements, the Company was required to recognize the initial consideration, consisting of the $45.0 million of nonrefundable upfront license and option payments, over the period the undelivered elements were to be delivered, which was initially estimated to be 15 years. As of the date of the modification of the agreement, there was approximately $34.7 million of deferred revenue under the arrangement.

	As a result of the material modifications to the cost sharing obligations, milestone payments structure and royalty payment structure, the Company concluded the modification represented a significant modification under ASU 2009-13, which required the Company to apply the updated provisions of ASU 2009-13 subsequent to the modification.

	Because the ACE-536 Agreement and the amendment to the Sotatercept Agreement were negotiated in contemplation of each other, and the Company had not yet completed all of its obligations pursuant to the Sotatercept Agreement, the agreements were considered one arrangement for accounting purposes. The deliverables under the combined arrangement include: (1) licenses to develop and commercialize sotatercept and ACE-536, (2) performance of research and development services, (3) participation on the joint development committees, and (4) the performance of manufacturing services to provide clinical material to Celgene. The Company has determined the option to future products related to the treatment of anemia represents a substantive option. The Company is under no obligation to discover, develop or deliver any new compounds that modulate anemia and Celgene is not contractually obligated to exercise the option. As a result, the Company is at risk as to whether Celgene will exercise the option.

	All of these deliverables identified in the arrangement were deemed to have stand-alone value and to meet the criteria to be accounted for as separate units of accounting under ASC 605-25. Factors considered in making this determination included, among other things, the subject of the licenses, the nature of the research and development services, and the capabilities of Celgene.

	The total arrangement consideration of $77.7 million under the ACE-536 Agreement and amended Sotatercept Agreement consisted of (1) the $25.0 million up-front payment for the license of ACE-536, (2) the remaining deferred revenue from the Sotatercept Agreement of $34.7 million, and (3) estimated payments for development activities and manufacturing services of $18.0 million. The Company used its BESP for each of the undelivered elements as the Company did not have VSOE or TPE of selling price for each deliverable. The Company's BESP considered its development plan for the compounds, expected manufacturing services, and reimbursement from Celgene (reimbursement of more than half of development expenses through December 31, 2012 and 100% thereafter). The Company determined its BESP for each of the undelivered elements under the arrangements as of the arrangement execution date as follows:



	•
	$18.8 million for research and development services

	•
	$2.9 million for the sotatercept joint development committee

	•
	$3.7 million for the ACE 536 joint development committee

•
$2.8 million for the manufacturing services

After determining the BESP of the undelivered elements, the remaining consideration of $49.5 million was recognized upon execution of the arrangements. The difference between the estimated payments of $18.0 million and the estimated selling prices which totaled $28.2 million, using BESP, for undelivered elements was $10.2 million. This amount was deferred at inception and will be recognized as the undelivered elements are delivered, using the proportional performance method, or ratably in the case of performance on the joint development committees.

As noted above, the total arrangement consideration includes estimated payments for development activities and manufacturing services identified at the outset of the ACE-536 Agreement and amended Sotatercept Agreement. At the end of each reporting period, the Company reassesses the estimated payments to be received related to these services and the BESP of the undelivered elements based upon the Company's current estimates. The Company accounts for such changes as a change in accounting estimate and the cumulative impact of any change is reflected in the period of change.

During 2011, the Company achieved a $7.5 million clinical milestone under its ACE-536 Agreement, related to the dosing of the first patient in a multiple-dose clinical trial. The Company evaluated the milestone and determined that it was not substantive, as there was no significant uncertainty to achieving the milestone upon execution of the ACE-536 Agreement. As such, the Company allocated the $7.5 million payment based on the allocation of arrangement consideration determined at the execution date of the ACE-536 Agreement and amended Sotatercept Agreement. Based on this allocation, the Company recognized $4.8 million of the payment upon achievement, with the remaining $2.7 million recognized as revenue as the undelivered elements are delivered, consistent with the treatment of the up-front payment. During 2011, the Company achieved a $7.0 million clinical milestone under its Sotatercept Agreement, related to the dosing of the first patient for a Phase 2b clinical trial. The Company evaluated the milestone and deemed it to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, recognized the $7.0 million payment in revenue during the year ended in December 31, 2011. During January 2013, the Company achieved a $10.0 million clinical milestone under its ACE-536 Agreement, related to the dosing of the first patient for a Phase 2 clinical trial. The Company evaluated the milestone and deemed it to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, recognized the $10.0 million payment in revenue during the year ended December 31, 2013. The remaining development milestones under the ACE-536 and Sotatercept Agreements were deemed to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, the Company will recognize payments related to the achievement of such milestones, if any, when such milestone is achieved. Factors considered in this determination included scientific and regulatory risks that must be overcome to achieve the milestones, the level of effort and investment required to achieve each milestone, and the monetary value attributed to each milestone. During the years ended December 31, 2013 2012 and 2011, the Company recognized $2.6 million, $2 million and $54.8 million, respectively, of the total deferred revenue as license and milestone revenue in the accompanying consolidated statements of operations and comprehensive loss.

As noted above, under the terms of the ACE-536 Agreement the Company retains responsibility for certain research and development activities through the completion of Phase 1 and initial Phase 2 clinical trials, as well as manufacturing the clinical supplies for these studies. Celgene is responsible for the conduct of subsequent Phase 2 and Phase 3 clinical studies. In November, 2013, the Company agreed to conduct additional activities for the benefit of the ACE-536 program including certain clinical and non-clinical services such as multiple toxicology studies and associated assay development and sample testing, clinical extension studies, and market development work. These activities will be reimbursed under the same terms and rates of the existing Agreements. The Company evaluated the additional services to be provided and determined that as the Company is under no obligation to conduct these additional activities, these services do not represent a deliverable under or modification to the ACE-536 Agreement, but rather, represent a separate services arrangement which should be accounted for as the services are delivered.

Pursuant to the terms of the agreement, Celgene and the Company share development costs, with Celgene responsible for substantially more than half of the costs for sotatercept and ACE-536 until December 31, 2012 and 100% of the costs from January 1, 2013 and thereafter. Payments from Celgene with respect to research and development costs incurred by the Company are recorded as cost-sharing revenue. Payments by the Company to Celgene for research and development costs incurred by Celgene are recorded as a reduction to cost-sharing revenue. During the years ended December, 2013, 2012 and 2011, the Company recorded net cost-sharing revenue of $12.7 million, $2.9 million and $(0.1) million, respectively, which includes payments to Celgene of $0, $2.8 million and $2.8 million, respectively, which were recorded as contra-revenue.

Other Agreements

Shire License

In September 2010, the Company entered into a license and collaboration agreement granting Shire AG the exclusive right to develop, manufacture and commercialize ActRIIB compounds in territories outside North America. Shire also received the right to conduct research and manufacture commercial supplies in North America for ActRIIB compounds. The lead ActRIIB compound was designated ACE-031. Under the initial development plan, the companies share the costs associated with developing and commercializing ACE-031, in Duchenne Muscular Dystrophy. In September 2010, Shire made a nonrefundable, up-front license payment to the Company of $45.0 million. In accordance with the Company's revenue recognition policy prior to the adoption of ASU 2009-13, the up-front license payment of $45.0 million was deferred, and will be recognized as revenue ratably over three years, which represented the original period over which the Company expected to perform and deliver research and development and manufacturing services. On February 8, 2011, the FDA placed ACE-031 on clinical hold. The Company re-assessed the duration of its deliverables under the license agreement and estimated the new term to be approximately five years. The adjustment was treated as a change in accounting estimate with the remaining deferred revenue of $38.8 million at February 8, 2011, recognized prospectively over the new period of research and development and manufacturing services. In April 2013, the Company and Shire determined not to further pursue development of ACE-031 and Shire sent the Company a notice of termination for the ACE-031 collaboration. The collaboration terminated effective June 30, 2013. At December 31, 2012, the Company had classified the remaining deferred revenue as current in the consolidated balance sheet. Upon the effectiveness of the termination of the Shire Agreement in the second quarter of 2013, the Company accelerated the recognition of $22.4 million of remaining deferred revenue from upfront non-refundable payments received under the Shire Agreement as it had no further obligation for deliverables under the Shire Agreement. During the years ended December 31, 2013, 2012 and 2011, the Company recognized $24.3 million, $7.7 million and $8.4 million, respectively of the up-front, non-refundable payments as license and milestone revenue in the accompanying consolidated statements of operations and comprehensive loss.

The agreement also included contingent milestone payments, based on the achievement of development milestones totaling $223.8 million and commercial milestones of $228.8 million for ActRIIB compounds. The milestones under the Shire Agreement were deemed to be substantive and consistent with the definition of a milestone included in ASU 2010-17 and, accordingly, the Company recognized payments related to the achievement of such milestones, if any, when such milestone was achieved. Factors considered in this determination included scientific and regulatory risks that must be overcome to achieve the milestones, the level of effort and investment required to achieve each milestone, and the monetary value attributed to each milestone.

Pursuant to the terms of the agreement, Shire and the Company shared development costs, with Shire responsible for 65% of the costs for ACE-031 and 55% of the costs for licensed compounds other than ACE-031. Payments from Shire with respect to research and development costs incurred by the Company are recorded as cost-sharing revenue. Payments by the Company to Shire for research and development costs incurred by Shire are recorded as a reduction to cost-sharing revenue. During the years ended December 31, 2013, 2012 and 2011, the Company recorded net cost-sharing revenue of $0.6 million, $2.7 million and $4.1 million, respectively, which includes payments to Shire of $0.2 million, $0.7 million and $2.0 million, respectively, which are recorded as contra-revenue in the accompanying consolidated statements of operations and comprehensive loss.

Alkermes License

In December 2009, the Company entered into a Collaboration and License Agreement with Alkermes Inc. (Alkermes) relating to a proprietary technology platform for extending the circulating half-life of certain proteins. Under the terms of the agreement, Alkermes paid the Company an up-front cash payment of $2.0 million in December 2009, which was deferred and recognized as license revenue ratably over the estimated research and development term. In addition, Alkermes purchased 636,942 shares of Series D-1 Preferred Stock at a per share price of $12.56, resulting in gross proceeds to the Company of $8.0 million. The Company determined that the price of $12.56 paid by Alkermes included a premium of $2.32 per share over the fair value of the Series D-1 Preferred Stock of $10.24 as calculated by the Company in its contemporaneous stock valuation. Accordingly, the Company has recognized the premium of $1.5 million as additional license revenue on a straight-line basis over the term discussed above. In October 2011, Alkermes discontinued development of the compounds being investigated under the license agreement, and as a result, the Company recognized the remaining $2.4 million of the up-front payment as revenue, as it had no further obligations under the arrangement, though the license continues.

As the principal in the collaboration, Acceleron recognized cost-sharing revenue for reimbursement payments from Alkermes. During the year ended December 31, 2011, the Company recognized net cost-sharing revenue of $0.7 million. No amounts were recognized in subsequent periods.

ImmunoGen Services Agreement

In October 2010, the Company entered into a Biopharmaceutical Services Agreement with ImmunoGen, Inc. Acceleron agreed to develop and manufacture an ImmunoGen product. The Company determined the arrangement should be accounted for as a service arrangement, using the proportional performance method. Accordingly, the Company recognized revenue as the underlying performance criteria were met. The costs associated with the services were charged to operations as incurred. As of December 31, 2011, the work was completed and the Company recorded revenue of $1.7 million for the year ended December 31, 2011.

Other

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. As compensation for the licenses, the Company issued 62,500 shares of its common stock to the institution, the fair value of which was $25,000, and was expensed during 2004, to research and development expense. The Company also agreed to pay specified development milestone payments totaling up to $2.0 million for sotatercept and $0.7 million for ACE-536. 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 a royalty ranging from 1.0%-3.5% of net sales on any products developed under the licenses. During the years ended December 31, 2013, 2012 and 2011, the Company paid and expensed milestones and fees defined under the agreement totaling $0.5 million, $0 and $0.5 million, respectively.

The Company entered into another license agreement with certain individuals for an exclusive, sublicensable, worldwide, royalty-bearing license to certain patents developed by the individuals. We agreed to pay specified development and sales milestone payments aggregating up to $1.0 million relating to the development and commercialization of dalantercept. In addition, we are required to pay royalties in the low single-digits on worldwide net product sales of dalantercept, with royalty obligations continuing at a 50% reduced rate for a period of time after patent expiration. If we sublicense our patent rights, we will owe a percentage of sublicensing revenue, excluding payments based on the level of sales, profits or other levels of commercialization. During the years ended December 31, 2013, 2012 and 2011, the Company did not reach any milestones defined under the agreement and, therefore, no amounts have been paid or expensed.

During 2012, the Company executed a license agreement with a research institution for an exclusive, sublicensable, worldwide, royalty-bearing license. The Company is obligated to pay development milestones and commercial milestone fees totaling up to $1.0 million. Under the agreement, if the Company uses the inventors in the clinical research, the development milestones are waived and commercial milestones shall change to $0.8 million plus any waived milestones. The Company will also pay $25,000 annually upon first commercial sale as well as royalties of 1.5% of net sales on any products developed under the patents. During the years ended December 31, 2013 and 2012, the Company did not reach any milestones defined under the agreement and, therefore, no amounts have been paid or expensed.

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
Stock-Based Compensation	Dec. 31, 2013
Stock-Based Compensation	'
Stock-Based Compensation	'

	10. Stock-Based Compensation

	At December 31, 2013, the Company had two stock-based compensations plans, which are more fully described below.

	The Company's 2003 Stock Option and Restricted Stock Plan (the 2003 Plan) provides for the issuance of stock options, restricted stock awards, and restricted stock to employees, officers, directors, consultants and key personnel of the Company as determined by the Board. In conjunction with the effectiveness of the 2013 Equity Incentive Plan (the 2013 Plan) described below, the Company determined that no further stock options or other equity-based awards may be granted under the 2003 Plan.

	On September 4, 2013, the Board and stockholders approved the adoption of the 2013 Equity Incentive Plan (the 2013 Plan). The Company has reserved for issuance an aggregate of 1,500,000 shares of common stock under the 2013 Plan which is comprised of (i) the remaining 155,884 shares reserved for issuance under the 2003 Plan and (ii) an additional 1,344,116 shares. The 2013 Plan provides that the number of shares reserved and available for issuance under the plan will automatically increase each January 1, beginning in 2014, by the lesser of (i) 3,150,000 shares, or (ii) 4% of the outstanding number of shares of the Company's common stock on the immediately preceding December 31st. This number is subject to adjustment in the event of a stock split, stock dividend or other change in the Company's capitalization. The number of options available for future grant was 956,000 at December 31, 2013.

	The Company has not granted unrestricted stock awards under the 2003 Plan and the 2013 Plan since its inception. Stock options carry an exercise price equal to the estimated fair value of the Company's common stock on the date of grant. Options generally expire ten years following the date of grant. Stock options and restricted stock awards typically vest over four years, but vesting provisions can vary based on the discretion of the Board.

	Shares of the Company's common stock underlying any awards that are forfeited, canceled, withheld upon exercise of an option, or settlement of an award to cover the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of shares of the Company's common stock, or otherwise terminated other than by exercise will be added back to the shares of common stock available for issuance under the 2013 Plan. Shares available for issuance under the 2013 Plan may be authorized but unissued shares of the Company's common stock or shares of the Company's common stock that have been reacquired by the Company.

	Additionally, on September 4, 2013, the Company's board of directors and stockholders approved the adoption of the 2013 Employee Stock Purchase Plan (the 2013 ESPP). Under the 2013 ESPP, 275,000 shares of the Company's common stock will be available for issuance and eligible employees of the Company may purchase shares of common stock during pre-specified purchase periods at a price equal to the lesser of 85% of the fair market value of a share of its common stock at the beginning of the purchase period or 85% of the fair market value of a share of its common stock at the end of the purchase period. The Board has not determined the date on which the initial purchase period will commence under the 2013 ESPP. As of December 31, 2013, the initial purchase period under the 2013 ESPP has not yet commenced.

	During 2010, the Company modified the awards of three employees that left the Company. The modifications all related to the term of vested options post termination. The changes ranged from 3.5 years to the remaining life of the option. Awards were reviewed under ASC 718, and the fair value of the unvested options that were modified will be re-measured and the expense adjusted at each reporting period. During the years ended December 31, 2013, 2012 and 2011 non-employee stock compensation expense of $3,000, $36,000, and $0.2 million respectively, was recorded.

	The Company recognized stock-based compensation expense under the various Plans totaling $2.2 million, $1.2 million and $1.4 million, during the years ended December 31, 2013, 2012 and 2011, respectively.

	Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations and comprehensive (loss) income is as follows (in thousands):


			Year Ended December 31,
			2013			2012			2011
	Research and development		$	659		$	514		$	686
	General and administrative			1,537			692			741


			$	2,196		$	1,206		$	1,427





	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:


			Year Ended December 31,
			2013			2012			2011
	Expected volatility			71.5	%		69	%		66	%
	Expected term (in years)			6			6			6
	Risk-free interest rate			1.85	%		0.9	%		1.1	%
	Expected dividend yield			—	%		—	%		—	%

	Fair Value of Underlying Instrument

	The Company estimates the fair value of its stock-based awards to employees using the Black-Scholes option pricing model.

	Expected Volatility

	The Company estimated the expected volatility based on actual historical volatility of the stock price of similar companies with publicly-traded equity securities. The Company calculated the historical volatility of the selected companies by using daily closing prices over a period of the expected term of the associated award. The companies were selected based on their enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the associated award. A decrease in the selected volatility would decrease the fair value of the underlying instrument.

	Expected Term

	The Company estimates the expected life of its employee stock options using the "simplified" method, as prescribed in Staff Accounting Bulletin (SAB) No. 107, whereby, the expected life equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data.

	Risk-Free Interest Rate

	The Company estimated the risk-free interest rate in reference to the yield on U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. A decrease in the selected risk-free rate would decrease the fair value of the underlying instrument.

	Expected Dividend Yield

	The Company estimated the expected dividend yield based on consideration of its historical dividend experience and future dividend expectations. The Company has not historically declared or paid dividends to stockholders. Moreover, it does not intend to pay dividends in the future, but instead expects to retain any earnings to invest in the continued growth of the business. Accordingly, the Company assumed an expected dividend yield of 0.0%.

Stock Option Activity

The following table summarizes the stock option activity under the Company's stock option plans during the year ended December 31, 2013 (in thousands, except per share amounts and years):


		Number			Weighted-			Weighted-			Aggregate
		of Grants			Average			Average			Intrinsic
					Exercise			Contractual			Value(1)
					Price Per			Life (in years)
					Share
Outstanding at December 31, 2012			3,730		$	4.16			6.62		$	13,946
Granted			553		$	23.88
Exercised			(293	)	$	2.24
Canceled or forfeited			(48	)	$	4.39


Outstanding at December 31, 2013			3,942		$	7.05			6.43		$	128,334
Exercisable at December 31, 2013			2,544		$	4.07			5.13		$	90,401




Vested and expected to vest at December 31, 2013(2)			3,860		$	6.89			6.37		$	126,258






-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 December 31, 2013 and 2012.

-2
This represents the number of vested options at December 31, 2013, plus the number of unvested options expected to vest at December 31, 2013, based on the unvested options outstanding at December 31, 2013, adjusted for the estimated forfeiture rate.

During the years ended December 31, 2013, 2012 and 2011, the Company granted stock options to purchase an aggregate of 552,750, 722,000 and 334,175 shares of its common stock, respectively, with a weighted-average grant date fair values of options granted of $15.27, $7.20, $5.12, respectively.

During the years ended December 31, 2013, 2012 and 2011, current and former employees of the Company exercised a total of 292,802, 38,697 and 94,748 options, respectively, resulting in total proceeds of $0.7 million, $0.2 million, and $0.2 million, respectively.

The aggregate intrinsic value of options exercised during the years ended December 31, 2013, 2012 and 2011, under the Company's stock option plans, was $7 million, $47,000 and $ 186,000, respectively.

As of December 31, 2013, there was $10.1 million of unrecognized compensation expense related to unvested stock options, under the Company's stock option plans, that is expected to be recognized over a weighted-average period of 2.66 years.

401k_Savings_Plan

401(k) Savings Plan	12 Months Ended
401(k) Savings Plan	Dec. 31, 2013
401(k) Savings Plan	'
401(k) Savings Plan	'

	11. 401(k) Savings Plan

	In 2004, the Company established a defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (the 401(k) Plan). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company has not made any contributions to the 401(k) Plan to date.

Income_Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2013
Income Taxes	'
Income Taxes	'

	12. Income Taxes

	The Company provides for income taxes under ASC 740. Under ASC 740, the liability method is used in accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities, and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse.

	For the years ended December 31, 2013, 2012 and 2011, the Company did not record a current or deferred income tax expense or benefit.

	The Company's (loss) income before income taxes was $(21.9) million, $(32.6) million and $36.3 million for the years ended December 31, 2013, 2012 and 2011, respectively, and was generated entirely in the United States.

	Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company's deferred tax assets are comprised of the following (in thousands):


		Year Ended December 31,
		2013			2012
	Deferred tax assets:
	U.S. and state net operating loss carryforwards	$	51,886		$	35,584
	Research and development credits		5,519			5,384
	Deferred revenue		3,005			21,882
	Accruals and other temporary differences		5,675			5,333


	Total deferred tax assets		66,085			68,183
	Less valuation allowance		(66,085	)		(68,183	)


	Net deferred tax assets	$	—		$	—





	The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company's history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of December 31, 2013 and 2012. The valuation allowance decreased by $2.1 million during the year ended December 31, 2013, due primarily to the decrease in deferred revenue during the period. The valuation allowance increased by $11.2 million during the year ended December 31, 2012, due primarily to the generation of net operating losses during the period.

	A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:


		Year Ended December 31,
		2013			2012			2011
	Federal income tax expense at statutory rate		34	%		34	%		34	%
	State income tax, net of federal benefit		(1.5	)%		4.2	%		5	%
	Permanent differences		(43.1	)%		(3.4	)%		1.5	%
	Research and development credit		0.7	%		—	%		(1.0	)%
	Other		0	%		(0.4	)%		—	%
	Change in valuation allowance		9.9	%		(34.4	)%		(39.5	)%




	Effective income tax rate		0	%		0	%		0	%





	As of December 31, 2013 and 2012, the Company had U.S. federal net operating loss carryforwards of $141.0 million and $93.3 million, respectively, which may be available to offset future income tax liabilities and expire at various dates through 2033. As of December 31, 2013 and 2012, the Company also had U.S. state net operating loss carryforwards of $122.9 million and $75.4 million, respectively, which may be available to offset future income tax liabilities and expire at various dates through 2033.

	As of December 31, 2013 and 2012, the Company had federal research and development tax credit carryforwards of $3.9 million and $3.8 million, respectively, available to reduce future tax liabilities which expire at various dates through 2033. As of December 31, 2013 and 2012, the Company had state research and development tax credit carryforwards of approximately $2.4 million and $2.4 million, respectively, available to reduce future tax liabilities which expire at various dates through 2028.

	Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50 percent, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception which may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code, or could result in a change in control in the future. The Company has not, as of yet, conducted a study to determine if any such changes have occurred that could limit its ability to use the net operating loss and credit carryforwards.

	The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2013 and 2012, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company's consolidated statements of operations and comprehensive (loss) income.

	For all years through December 31, 2013, the Company generated research credits but has not conducted a study to document the qualified activities. This study may result in an adjustment to the Company's research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position for these two years. A full valuation allowance has been provided against the Company's research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carryforwards and the valuation allowance.

The Company files income tax returns in the United States, and various state jurisdictions. The federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2010 through December 31, 2013. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, state or foreign tax authorities to the extent utilized in a future period.

LongTerm_Debt

Long-Term Debt	12 Months Ended
Long-Term Debt	Dec. 31, 2013
Long-Term Debt.	'
Long-Term Debt	'

	13. Long-Term Debt

	On June 7, 2012, the Company entered into a loan and security agreement (the Loan Agreement) with three lenders, pursuant to which the Company received a loan in the aggregate principal amount of $20.0 million. The Company is required to repay the aggregate principal balance under the Loan Agreement in 42 months. The first 12 payments are interest only and the remaining 30 payments are equal monthly installments of principal plus interest. The Loan Agreement provided that the interest only period could be extended under certain circumstances. The Company did not trigger the requirements and began paying principal in July 2013.

	Per annum interest is payable at the 8.5%. The Loan Agreement also included a closing fee of $0.2 million. The Company is amortizing the cost over the 42 months of loan. The Loan Agreement is also subject to an additional deferred payment of $1.2 million due with the final payment. The Company is recording the deferred payment to interest expense over the term of the Loan Agreement. The resulting effective interest rate is approximately 11.8%. The Loan Agreement is secured by a lien on all of the Company's personal property as of, or acquired after, the date of the Loan Agreement, except for intellectual property.

	The Loan Agreement defines events of default, including the occurrence of an event that results in a material adverse effect upon the Company's business operations, properties, assets or condition (financial or otherwise), its ability to perform its obligations under and in accordance with the terms of the Loan Agreement, or upon the ability of the lenders to enforce any of their rights or remedies with respect to such obligations, or upon the collateral under the Loan Agreement or upon the liens of the lenders on such collateral or upon the priority of such liens. The lenders also received a right, to purchase at fair value, up to $2.0 million of equity of the Company sold in any sale by the Company to third parties of equity securities resulting in at least $5.0 million in net cash proceeds to the Company, subject to certain exceptions. On December 30, 2013, in conjunction with the establishment of the Company's wholly owned subsidiary, Acceleron Security Corporation Inc., the Loan Agreement was modified to add a debt covenant that requires the Company to maintain a cash balance equal to 150% of the outstanding principal debt balance in a separate bank account, at all times. That bank account can be subject to the lender's exclusive control in the event of a default. As of December 31, 2013 and 2012, there have been no events of default under the loan and the Company was in compliance with all financial and nonfinancial covenants at December 31, 2013. As of December 31, 2013 and 2012, the principal balance outstanding was $16.3 million and $20.0 million, respectively.

	On March 12, 2014, the Company paid off the remaining principal outstanding under the Loan Agreement. In accordance with ASC 470-10-45 Intent and Ability to Refinance on A Long Term Basis, the Company has classified the outstanding debt balance as a current liability as of December 31, 2013.

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2013
Related Party Transactions	'
Related Party Transactions	'

	14. Related Party Transactions

	Celgene Corporation (Celgene)

	In connection with its entry into the collaboration agreement with Celgene, on February 2008, the Company sold Celgene 457,875 shares of its Series C-1 Preferred Stock. As part of the Company's June 2010 Series E financing, Celgene purchased 36,496 shares of Series E Preferred Stock and received warrants to purchase 38,979 shares of common stock. As part of the Company's December 2011 Series F financing, Celgene purchased 1,990,446 shares of Series F Preferred Stock. In connection with the Company's IPO, Celgene purchased 666,667 shares of common stock. As a result of these transactions, Celgene owned 9.7% and 9.9% of the Company's fully diluted equity as of December 31, 2013 and 2012, respectively. Refer to Note 9 for additional information regarding this collaboration agreement.

	During the year ended December 31, 2013, the Company recognized $32.3 million in collaboration revenue under the Celgene collaboration arrangement and, as of December 31, 2013, had $7.7 million of deferred revenue related to the Celgene collaboration arrangement. During the year ended December 31, 2012, the Company recognized $4.9 million in collaboration revenue under the Celgene collaboration arrangement and, as of December 31, 2012, had $10.3 million of deferred revenue related to the Celgene collaboration arrangement. During the year ended December 31, 2011, the Company recognized $63.5 million in collaboration revenue under the Celgene collaboration arrangement.

	The Company recognized revenue from Celgene during the years ended December 31, 2013, 2012 and 2011 as follows (in thousands):


		Year Ended December 31,
		2013		2012		2011
	License and milestone	$	19,626	$	2,035	$	63,607
	Cost sharing, net		12,658		2,879		(121	)


		$	32,284	$	4,914	$	63,486





	Alkermes

	One of the Company's directors is also the Chairman, President, and Chief Executive Officer of Alkermes plc, the parent company of Alkermes, with which the Company entered into a collaboration agreement during 2009.

	As of December 31, 2013, Alkermes held 718,655 common shares of the Company's (upon conversion of its investment in the Company's Preferred stock) and warrants to purchase 42,624 shares of common stock. As of December 31, 2012, Alkermes held 695,250 shares of the Company's Preferred Stock and warrants to purchase 42,624 shares of common stock. Upon the closing of the IPO on September 24, 2013, all of the shares of the Company's preferred stock held by Alkermes were converted into 718,655 shares of common stock.

	For the year ended December 31, 2011, Alkermes paid the Company $0.7 million for research services. No such fees were paid to the Company during 2012 or 2013.

	Related-Party Receivable

	On January 28, 2008, the Company issued a secured promissory note (the Note Receivable) in the amount of $0.2 million to the current chief executive officer of the Company (the CEO). The Note Receivable bore interest at an annual interest rate of 3.11% and was initially repayable on the earlier of January 28, 2011, or the date prior to the date that the Company files a registration statement with the SEC, covering shares of its common stock. The Note Receivable was secured by shares of the Company's common stock owned by the CEO. On December 22, 2010, the term was extended until January 28, 2014, or the date prior to the date that the Company files a registration statement with the SEC covering shares of its common stock.

	In November 2012, the Company further modified the terms of the Note Receivable, such that in the event that an acquisition event occurs or the Company files a registration statement with the SEC on or before the maturity date, the unpaid principal and interest will be forgiven. The Company evaluated the forgiveness provisions and determined that forgiveness was not probable as of December 31, 2012, and as such, continued to record the Note Receivable as an asset at December 31, 2012. As a result of the Company's filing of a registration statement with the SEC on August 6, 2013 which triggered the forgiveness of the Note Receivable, the Company expensed the unpaid principal and interest expense totaling $0.2 million as compensation expense during the year ended December 31, 2013.

Subsequent_Events

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2013
Subsequent Events	'
Subsequent Events	'

	15. Subsequent Events

	The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other the following:

	On January 28, 2014, the Company completed a follow-on public offering of its common stock and issued and sold 2,760,000 shares of our common stock, including 360,000 shares of common stock sold pursuant to the underwriters' full exercise of their option to purchase additional shares at a public offering price of $50.00 per share, for aggregate net proceeds received by the Company were $129.0 million, after deducting underwriting discounts of $8.3 million and offering expenses totaling $0.7 million. All of the shares issued and sold in this follow-on public offering were registered under the Securities Act pursuant to a Registration Statement on Form S-1 (File No. 333-193252), which was declared effective by the SEC on January 22, 2014, and a registration statement filed pursuant to Rule 462(b) of the Securities Act.

	On March 12, 2014, the Company paid off the remaining principal outstanding under the Loan Agreement (see Note 13 Long-term Debt). Accordingly, the Company has classified the outstanding debt balance as a current liability as of December 31, 2013.

Quarterly_Financial_Data_unaud

Quarterly Financial Data (unaudited)	12 Months Ended
Quarterly Financial Data (unaudited)	Dec. 31, 2013
Quarterly Financial Data (unaudited)	'
Quarterly Financial Data (unaudited)	'

	16. Quarterly Financial Data (unaudited)

	The following table presents certain unaudited quarterly financial information for the eight quarters in the period ended December 31, 2013. This information has been prepared on the same basis as the audited financial statements and includes all adjustments (consisting only of normal recurring adjustments) necessary to present fairly the unaudited quarterly results of operations set forth herein. Net income (loss) per share for all periods presented have been retroactively adjusted to reflect the 1-for-4 reverse stock split effected on September 5, 2013.


		For the Three Months Ended(1)
		March 31			June 30			September 30			December 31
		(in thousands except per share data)
	2013:00:00
	Total revenue	$	15,012		$	26,427		$	4,270		$	11,521
	Total costs and expenses		(11,876	)		(12,276	)		(11,154	)		(14,972	)
	Income (loss) from operations		3,136			14,151			(6,884	)		(3,451	)
	Net income (loss)		1,647			13,078			(18,513	)		(18,110	)
	Basic net income (loss) per share*	$	(0.24	)	$	0.3		$	(5.62	)	$	(0.64	)
	Diluted net income (loss) per share*	$	(0.24	)	$	0.28		$	(5.62	)	$	(0.64	)
	2012:00:00
	Total revenue	$	3,324		$	4,040		$	3,905		$	3,985
	Total costs and expenses		(10,257	)		(10,944	)		(10,763	)		(12,179	)
	Loss from operations		(6,933	)		(6,904	)		(6,858	)		(8,194	)
	Net loss		(7,588	)		(7,400	)		(7,215	)		(10,379	)
	Basic net loss per share*	$	(1.50	)	$	(5.93	)	$	(5.82	)	$	(7.10	)
	Diluted net loss per share*	$	(1.50	)	$	(5.93	)	$	(5.82	)	$	(7.10	)


	-1
	The amounts were computed independently for each quarter, and the sum of the quarters may not total the annual amounts.

	*
	Applicable to common stockholders

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2013

Summary of Significant Accounting Policies

Principles of Consolidation

The accompanying consolidated financial statements include those of the Company and its wholly-owned subsidiary, Acceleron Pharma Security Corporation. All significant intercompany balances and transactions have been eliminated in consolidation.

Basis of Presentation

The accompanying 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).

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. Actual results could materially differ from those estimates.

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: revenue recognition, stock-based compensation expense, the determination of the fair value of stock-based awards, the fair value of liability-classified warrants, accrued expenses, and the recoverability of the Company's net deferred tax assets and related valuation allowance.

The Company utilized significant estimates and assumptions in determining the fair value of its common stock, prior to the completion of its IPO. The Company's board of directors (the Board) determined the estimated fair value of the Company's common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector and the prices at which the Company sold shares of redeemable convertible preferred stock, the superior rights and preferences of securities senior to the Company's common stock at the time, and the likelihood of achieving a liquidity event, such as an IPO or sale of the Company.

Collaboration Receivable

Credit is extended to customers based upon an evaluation of the customer's financial condition. Collaboration receivables are recorded at net realizable value. The Company does not charge interest on past due balances. Collaboration receivables are determined to be past due when the payment due date is exceeded. The Company utilizes a specific identification accounts receivable reserve methodology based on a review of outstanding balances and previous activities to determine the allowance for doubtful accounts. The Company charges off uncollectible receivables at the time the Company determines the receivable is no longer collectible. The Company did not have an allowance for doubtful accounts at December 31, 2013 or 2012.

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 operating segment, which is the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. All material long-lived assets of the Company reside in the United States. The Company does use contract research organizations (CROs) and research institutions located outside the United States. Some of these expenses are subject to collaboration reimbursement which is presented as a component of cost sharing, net in the statement of operations and comprehensive income (loss).

Cash and Cash Equivalents

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 market value.

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 and accounts receivable. The Company maintains its cash and cash equivalent balances in the form of money market accounts with financial institutions that management believes are creditworthy. 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 concentration of credit risk.

The Company routinely assesses the creditworthiness of its customers and collaboration partners. 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 additional credit risk beyond amounts provided for collection losses is believed by management to be probable in the Company's accounts receivable.

Disclosure of Fair Value of Financial Instruments

The carrying amounts of the Company's financial instruments, which include cash, cash equivalents, collaboration receivables, accounts payable, accrued expenses and notes payable, approximated their fair values at December 31, 2013 and 2012, due to the short-term nature of these instruments, and for the notes payable, the interest rates the Company believes it could obtain for borrowings with similar terms. See discussion below on the determination of the fair value of the Company's preferred and common stock warrants.

The Company has evaluated the estimated fair value of financial instruments using available market information and management's estimates. The use of different market assumptions and/or estimation methodologies could have a significant effect on the estimated fair value amounts.

Fair Value Measurements

ASC Topic 820, Fair Value Measurement (ASC 820), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company's own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company's assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC Topic 820 establishes a three-tier fair value hierarchy that distinguishes between the following:

•

Level 1—Quoted market prices in active markets for identical assets or liabilities.

•

Level 2—Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves.

•

Level 3—Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument's level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Items measured at fair value on a recurring basis include warrants to purchase redeemable convertible preferred stock, which were outstanding until the closing of the IPO, and warrants to purchase common stock (Note 6). 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 tables set forth the Company's financial instruments carried at fair value using the lowest level of input applicable to each financial instrument as of December 31, 2013 and 2012 (in thousands):

December 31, 2013

Quoted Prices

Significant Other

Significant

Total

in Active Markets

Observable

Unobservable

for Identical Items

Inputs

(Level 1)

(Level 2)

(Level 3)

Assets:

Money market funds

101,394

—

101,394

Restricted cash

913

—

913

Total assets

102,307

—

102,307

Liabilities:

Warrants to purchase common stock

—

30,753

Total liabilities

—

30,753

December 31, 2012

Quoted Prices

Significant other

Significant

Total

in Active Markets

Observable

Unobservable

for Identical Items

Inputs

(Level 1)

(Level 2)

(Level 3)

Assets:

Money market funds

36,847

—

36,847

Restricted cash

913

—

913

Total assets

37,760

—

37,760

Liabilities:

Warrants to purchase redeemable convertible preferred stock

—

1,422

Warrants to purchase common stock

—

5,229

Total liabilities

—

6,651

The following table sets forth a summary of changes in the fair value of the Company's preferred and 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):

Year Ended

December 31,

2013

2012

Beginning balance

6,651

4,393

Change in fair value

26,875

2,258

Exercises

(678

)

—

Repurchases

(83

)

—

Conversions

(2,012

)

—

Ending balance

30,753

6,651

The money market funds noted above are included in cash and cash equivalents in the accompanying 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 years ended December 31, 2013 or 2012 except for the transfer out of the warrants to purchase redeemable convertible preferred stock as described below.

During the year 2013, as a result of the closing of the IPO, the warrants to purchase preferred stock were converted to warrants to purchase common stock. The resulting warrants to purchase common stock meet the criteria to be classified as permanent equity and are no longer required to be measured at fair value at each reporting period.

The fair value of the warrants to purchase preferred stock that were classified as liabilities was estimated using the Black-Scholes option pricing model at the date of issuance and on each re-measurement date. This method of valuation involves using inputs such as the fair value of the Company's various classes of preferred stock, stock price volatility, the contractual term of the warrants, risk free interest rates, and dividend yields. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement. See Note 6 for further discussions of the accounting for the warrants, as well as for a summary of the significant inputs and assumptions used to determine the fair value of the warrants.

The fair value of warrants to purchase common stock that are classified as liabilities was estimated using a Monte Carlo model up until September 30, 2013. This method of valuation involves using inputs such as the fair value of a share of common stock, stock price volatility, and the contractual term of the warrants. Due to the warrants being deeply in the money as of December 31, 2013, the valuation method to estimate the warrants to purchase common stock was changed to the Black-Scholes option pricing model. Due to the nature of these inputs, the valuation of the warrants is considered a Level 3 measurement.

The Company measures eligible assets and liabilities at fair value, with changes in value recognized in earnings. Fair value treatment may be elected either upon initial recognition of an eligible asset or liability or, for an existing asset or liability, if an event triggers a new basis of accounting. The Company did not elect to remeasure any of its existing financial assets or liabilities, and did not elect the fair value option for any financial assets and liabilities transacted in the years ended December 31, 2013 or 2012.

Property and Equipment

Property and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows:

Asset

Estimated Useful Life

Computer equipment and software

3 years

Office and laboratory equipment

3 years

Leasehold improvements

Shorter of the useful life or remaining lease term

The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No impairment losses have been recorded during the years ended December 31, 2013, 2012 or 2011.

Revenue Recognition

The company has primarily generated revenue through collaboration, license and research arrangements with collaboration partners for the development and commercialization of protein therapeutics.

The Company recognizes revenue in accordance with FASB ASC Topic 605, Revenue Recognition. Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met: (1) persuasive evidence of an arrangement exists; (2) delivery has occurred or services have been rendered; (3) the fee is fixed or determinable; and (4) collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company's consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

Multiple Element Revenue Arrangements

The Company enters into collaboration agreements from time to time, which are more fully described in Note 9. The arrangements generally contain multiple elements or deliverables, which may include (1) licenses, or options to obtain licenses, to the Company's technology, (2) research and development activities performed for the collaboration partner, (3) participation on Joint Development Committees, and (4) manufacturing clinical or preclinical material. Payments pursuant to these arrangements typically include non-refundable, up-front payments, milestone payments upon achieving significant development events, research and development reimbursements, sales milestones, and royalties on future product sales.

Effective January 1, 2011, the Company adopted ASU No. 2009-13, Multiple-Deliverable Revenue Arrangements (ASU 2009-13), which amends Topic 605-25, Revenue Recognition—Multiple Element Arrangements (ASC 605-25). The Company applies this guidance to new arrangements as well as existing agreements that are significantly modified after January 1, 2011. For agreements that are significantly modified, the Company determines the estimated selling price for the remaining undelivered elements as of the date of the material modification and allocates arrangement consideration based upon the estimated selling price to the undelivered elements.

The application of the multiple element guidance requires subjective determinations, and requires management to make judgments about the individual deliverables, and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that: (1) the delivered item(s) has value to the customer on a stand-alone basis and (2) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially in the control of the Company. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have stand-alone value, based on the consideration of the relevant facts and circumstances for each arrangement, such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use the other deliverable(s) for their intended purpose without the receipt of the remaining element(s), whether the value of the deliverable is dependent on the undelivered item(s) and whether there are other vendors that can provide the undelivered element(s). Arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria, as described above, are applied to each of the separate units of accounting in determining the appropriate period or pattern of recognition.

The Company determines the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (VSOE) of selling price, if available, third-party evidence (TPE) of selling price if VSOE is not available, or management's best estimate of selling price (BESP) if neither VSOE nor TPE is available. Subsequent to the adoption of ASU 2009-13, the Company typically uses BESP to estimate the selling price of the deliverables. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

The Company typically receives up-front, non-refundable payments when licensing its intellectual property in conjunction with a collaboration agreement. When management believes the license to its intellectual property does not have stand-alone value from the other deliverables to be provided in the arrangement, the Company generally recognizes revenue attributed to the license on a straight-line basis over the contractual or estimated performance period, which is typically the term of the Company's research and development or manufacturing obligations. The Company continually evaluates these periods, and will adjust the period of revenue recognition if circumstances change. When management believes the license to its intellectual property has stand-alone value, the Company generally recognizes revenue attributed to the license upon delivery.

Research and development funding is recognized as revenue in the period that the related services are performed. When the Company acts as the principal under its collaboration agreements, it records payments received for the reimbursement of research and development costs as cost-sharing revenue in the consolidated statements of operations and comprehensive income (loss). To the extent that the Company reimburses the collaborator for costs incurred, the Company records these costs as a reduction of cost-sharing revenue.

The Company's agreements may contain options which provide the collaboration partner the right to obtain additional licenses. Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors considered in evaluating whether an option is substantive include the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option, the cost to exercise the option and the likelihood that the option will be exercised. For arrangements under which an option is considered substantive, the Company does not consider the item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable arrangement consideration, assuming the option is not priced at a significant and incremental discount. Conversely, for arrangements under which an option is not considered substantive or if an option is priced at a significant and incremental discount, the Company would consider the item underlying the option to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration.

Effective January 1, 2011, the Company adopted ASU No. 2010-17, Revenue Recognition—Milestone Method (ASU 2010-17). At the inception of each arrangement that includes milestone payments, the Company evaluates, with respect to each milestone, whether the milestone is substantive and at-risk. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (1) the entity's performance to achieve the milestone, or (2) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting at least in part from the entity's performance to achieve the milestone, (b) the consideration relates solely to past performance, and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, regulatory, commercial, and other risks that must be overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone, and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. On the milestone achievement date, assuming all other revenue recognition criteria are met and the milestone is deemed substantive and at-risk, the Company recognizes the payment as license and milestone revenue. For milestones that are not deemed substantive and at-risk, where payment is reasonably assured, the Company recognizes the milestone payment over the remaining service period.

Sales and commercial milestones and royalties will be recognized when and if earned, provided collectability is reasonably assured.

Contract Manufacturing Revenue

Contract manufacturing revenue is recognized upon delivery of the product in accordance with the terms of the contract, which specifies when transfer of title and risk of loss occurs.

Research and Development Expenses

Research and development costs are charged to expense as costs are incurred in performing research and development activities. Research and development costs include all direct costs, including salaries, stock compensation and benefits for research and development personnel, outside consultants, costs of clinical trials, sponsored research, clinical trials insurance, other outside costs, depreciation and facility costs related to the development of drug candidates. The Company records up-front, non-refundable payments made to outside vendors, or other payments made in advance of services performed or goods being delivered, as prepaid expenses, which are expensed as services are performed or the goods are delivered.

Certain research and development projects are, or have been, partially funded by collaboration agreements, and the expenses related to these activities are included in research and development costs. The Company records the related reimbursement of research and development costs under these agreements as revenue, as more fully described above and in Note 9.

Stock-Based Compensation

At December 31, 2013, the Company had two stock-based compensation plans, which are more fully described in Note 10. The Company accounts for stock-based compensation in accordance with the provisions of ASC Topic 718, Compensation—Stock Compensation (ASC 718), which requires the recognition of expense related to the fair value of stock-based compensation awards in the statements of operations and comprehensive income (loss).

For stock options issued to employees and members of the Board for their services on the Board, the Company estimates the grant date fair value of each option using the Black-Scholes option-pricing model. The use of the Black-Scholes option-pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the common stock consistent with the expected life of the option, risk-free interest rates and expected dividend yields of the common stock. For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense, net of estimated forfeitures, equal to the grant date fair value of stock options on a straight-line basis over the requisite service period, which is generally the vesting term. For awards subject to both performance and service-based vesting conditions, the Company recognizes stock-based compensation expense using an accelerated recognition method when it is probable that the performance condition will be achieved. Forfeitures are required to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.

Share-based payments issued to non-employees are recorded at their fair values, and are periodically revalued as the equity instruments vest and are recognized as expense over the related service period in accordance with the provisions of ASC 718 and ASC Topic 505, Equity. For stock-based awards granted to non-employees, the Company recognizes stock-based compensation expense using an accelerated recognition method.

See Note 10 for a discussion of the assumptions used by the Company in determining the grant date fair value of options granted under the Black-Scholes option pricing model, as well as a summary of the stock option activity under the Company's stock-based compensation plan for the year ended December 31, 2013.

Income Taxes

Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (ASC 740), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2013 and 2012, the Company does not have any significant uncertain tax positions.

Net Income (Loss) Per Share

Net income (loss) per share information is determined using the two-class method, which includes the weighted-average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). The Company's redeemable convertible preferred stock are participating securities as defined by ASC 260-10, Earnings Per Share.

Under the two-class method, basic net income (loss) per share applicable to common stockholders is computed by dividing the net income (loss) applicable to common stockholders by the weighted-average number of common shares outstanding for the reporting period. Diluted net income (loss) per share is computed using the more dilutive of (1) the two-class method or (2) the if-converted method. The Company allocates net income first to preferred stockholders based on dividend rights under the Company's articles of incorporation and then to preferred and common stockholders based on ownership interests. Net losses are not allocated to preferred stockholders as they do not have an obligation to share in the Company's net losses.

Diluted net income (loss) per share gives effect to all potentially dilutive securities, including redeemable convertible preferred stock, and shares issuable upon the exercise of outstanding warrants and stock options, using the treasury stock method. For the years ended December 31, 2013 and 2012, the Company has excluded the effects of all potentially dilutive shares, which include redeemable convertible preferred stock, warrants for redeemable convertible preferred stock, warrants for common stock and outstanding common stock options, from the weighted-average number of common shares outstanding as their inclusion in the computation for these two years would be anti-dilutive due to net losses.

The following common stock equivalents were excluded from the calculation of diluted net income (loss) per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands):

Year Ended December 31,

2013

2012

2011

Outstanding stock options

3,709

3,730

—

Common stock warrants

908

884

874

Preferred stock

13,218

18,166

—

Preferred stock warrants

114

248

—

17,949

23,028

874

Comprehensive Income (Loss)

Comprehensive income (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 income (loss) consists of net income (loss) and other comprehensive income (loss), which includes certain changes in equity that are excluded from net income (loss). Comprehensive income (loss) has been disclosed in the accompanying consolidated statements of operations and comprehensive (loss) income and equals the Company's net income (loss) for all periods presented.

Subsequent Events

The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. See Note 15.

Application of New or Revised Accounting Standards

On April 5, 2012, the Jump-Start Our Business Startups Act (the JOBS Act) was signed into law. The JOBS Act contains provisions that, among other things, reduce certain reporting requirements for an "emerging growth company." As an emerging growth company the Company has elected to not take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards, and as a result, will comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non-emerging growth companies.

Recently Adopted Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
Summary of Significant Accounting Policies (Tables)	Dec. 31, 2013
Summary of Significant Accounting Policies	'
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 applicable to each financial instrument as of December 31, 2013 and 2012 (in thousands):


		December 31, 2013
		Quoted Prices			Significant Other		Significant		Total
		in Active Markets			Observable		Unobservable
		for Identical Items			Inputs		Inputs
		(Level 1)			(Level 2)		(Level 3)
	Assets:
	Money market funds	$	101,394		$	—	$	—	$	101,394
	Restricted cash		913			—		—		913




	Total assets	$	102,307		$	—	$	—	$	102,307




	Liabilities:
	Warrants to purchase common stock	$	—		$	—	$	30,753	$	30,753


	Total liabilities	$	—		$	—	$	30,753	$	30,753









		December 31, 2012
		Quoted Prices			Significant other		Significant		Total
		in Active Markets			Observable		Unobservable
		for Identical Items			Inputs		Inputs
		(Level 1)			(Level 2)		(Level 3)
	Assets:
	Money market funds	$	36,847		$	—	$	—	$	36,847
	Restricted cash		913			—		—		913




	Total assets	$	37,760		$	—	$	—	$	37,760




	Liabilities:
	Warrants to purchase redeemable convertible preferred stock	$	—		$	—	$	1,422	$	1,422
	Warrants to purchase common stock		—			—		5,229		5,229


	Total liabilities	$	—		$	—	$	6,651	$	6,651




Summary of changes in the fair value of the preferred and common stock warrant liabilities	'

	The following table sets forth a summary of changes in the fair value of the Company's preferred and 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):


		Year Ended
		December 31,
		2013			2012
	Beginning balance	$	6,651		$	4,393
	Change in fair value		26,875			2,258
	Exercises		(678	)		—
	Repurchases		(83	)		—
	Conversions		(2,012	)		—


	Ending balance	$	30,753		$	6,651




Schedule of estimated useful lives of the assets	'






	Asset	Estimated Useful Life
	Computer equipment and software	3 years
	Office and laboratory equipment	3 years
	Leasehold improvements	Shorter of the useful life or remaining lease term
Schedule of anti-dilutive common stock equivalents excluded from the calculation of diluted net income (loss) per share	'

	The following common stock equivalents were excluded from the calculation of diluted net income (loss) per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands):


		Year Ended December 31,
		2013			2012		2011
	Outstanding stock options		3,709			3,730		—
	Common stock warrants		908			884		874
	Preferred stock		13,218			18,166		—
	Preferred stock warrants		114			248		—


			17,949			23,028		874

Property_and_Equipment_Net_Tab

Property and Equipment, Net (Tables)	12 Months Ended
Property and Equipment, Net (Tables)	Dec. 31, 2013
Property and Equipment, Net	'
Schedule of property and equipment, net	'

	Property and equipment, net, consists of the following (in thousands):


		December 31,
		2013			2012
	Computer equipment and software	$	921		$	919
	Office equipment		205			179
	Laboratory equipment		12,334			11,815
	Leasehold improvements		9,930			10,088
	Construction in progress		—			8


	Total property and equipment		23,390			23,009
	Accumulated depreciation and amortization		(19,685	)		(18,950	)


	Property and equipment, net	$	3,705		$	4,059

Accrued_Expenses_Tables

Accrued Expenses (Tables)	12 Months Ended
Accrued Expenses (Tables)	Dec. 31, 2013
Accrued Expenses	'
Schedule of accrued expenses	'

	Accrued expenses consist of the following (in thousands):


		December 31,
		2013		2012
	Collaboration expense	$	—	$	1,000
	Research and development related		1,330		1,282
	Employee compensation		2,930		2,448
	Professional services		1,017		607
	Other		1,650		816


		$	6,927	$	6,153

Warrants_Tables

Warrants (Tables)	12 Months Ended
Warrants (Tables)	Dec. 31, 2013
Warrants	'
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
		Warrants as of										Balance Sheet
								Weighted-				Classification
								Average
		December 31,						Exercise
		December 31,						Price Per				December 31,	December 31,
		2013			2012			Share			Expiration	2013	2012
	Warrant to purchase Series A Preferred Stock		—			107		$	4		February 28, 2013	N/A(1)	Liability
	Warrants to purchase Series B Preferred Stock		—			32			7.4		December 21, 2013	N/A(2)(4)	Liability
	Warrants to purchase Series C-1 Preferred Stock		—			46			10.92		June 25, 2019	N/A(2)	Liability
	Warrants to purchase Series D-1 Preferred Stock		—			64			12.56		March 18, 2020	N/A(2)	Liability
	Warrants to purchase Common Stock		46			—			10.92		June 25, 2019	Equity(2)	N/A
	Warrants to purchase Common Stock		64			—			12.56		March 18, 2020	Equity(2)	N/A
	Warrants to purchase Common stock		857			872			5.88		June 10, 2020 - July 9, 2020	Liability	Liability
	Warrants to purchase Common stock		13			13			4.00 - 7.40		March 31, 2015 - December 31, 2017	Equity(3)	Equity


	All warrants		980			1,134		$	6.53






	-1
	On February 6, 2013, the warrant holder exercised a warrant to purchase 107 shares of Series A Preferred Stock on a net basis, resulting in the issuance of 47 shares of Series A Preferred Stock.

	-2
	Warrants to purchase Series B Preferred Stock, Series C-1 Preferred Stock, and Series D-1 Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

	-3
	Warrants to purchase common stock were issued in connection with various debt financing transactions that were consummated in periods prior to December 31, 2012. See discussion below for further details.

	-4
	On December 19 and 20, 2013, the warrants holders exercised warrants to purchase 32 shares of Common Stock on a net basis, resulting in the issuance of 24 shares of Common Stock.

Warrant to purchase Series A preferred stock	'
Warrants	'
Schedule of assumptions used in estimating fair value of warrants using the Black-Scholes option pricing model	'






		Year Ended December 31,
		2013(2)			2012(1)			2011
	Fair value of underlying instrument		n/a		$	9.24		$	6.76
	Expected volatility		n/a			69.1	%		66	%
	Expected term (in years)		n/a			0.16			1.16
	Risk-free interest rate		n/a			0.04	%		0.12	%
	Expected dividend yield		n/a			—	%		—	%


	-1
	During December 2012, the expiration date of the warrant to purchase Series A Preferred Stock was extended from December 21, 2012 to February 28, 2013.

	-2
	The warrant to purchase Series A Preferred Stock was exercised during the three months ended March 31, 2013.

Warrant to purchase Series B preferred stock	'
Warrants	'
Schedule of assumptions used in estimating fair value of warrants using the Black-Scholes option pricing model	'






		Year Ended December 31,
		2013(1)			2012			2011
	Fair value of underlying instrument	$	20.03		$	9.96		$	7.56
	Expected volatility		71.2	%		69.1	%		66	%
	Expected term (in years)		0.24			0.97			1.98
	Risk-free interest rate		0.04	%		0.16	%		0.25	%
	Expected dividend yield		—	%		—	%		—	%


	-1
	Warrants to purchase Series B Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

Warrant to purchase Series C-1 preferred stock	'
Warrants	'
Schedule of assumptions used in estimating fair value of warrants using the Black-Scholes option pricing model	'






		Year Ended December 31,
		2013(1)			2012			2011
	Fair value of underlying instrument	$	20.03		$	11.04		$	8.84
	Expected volatility		71.2	%		69.1	%		66	%
	Expected term (in years)		5.73			6.46			7.46
	Risk-free interest rate		1.75	%		0.95	%		1.35	%
	Expected dividend yield		—	%		—	%		—	%


	-1
	Warrants to purchase Series C-1 Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

Warrant to purchase Series D-1 preferred stock	'
Warrants	'
Schedule of assumptions used in estimating fair value of warrants using the Black-Scholes option pricing model	'






		Year Ended December 31,
		2013(1)			2012			2011
	Fair value of underlying instrument	$	20.03		$	10.52		$	8.84
	Expected volatility		71.2	%		69.1	%		66	%
	Expected term (in years)		6.48			7.22			8.22
	Risk-free interest rate		1.75	%		1.18	%		1.62	%
	Expected dividend yield		—	%		—	%		—	%


	-1
	Warrants to purchase Series D-1 Preferred Stock were converted to warrants to purchase common stock at the closing of the IPO on September 24, 2013.

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
Commitments and Contingencies (Tables)	Dec. 31, 2013
Commitments and Contingencies	'
Schedule of future annual minimum lease payments	'

	Future annual minimum lease payments as of December 31, 2013, are as follows (in thousands):


	2014	$	4,522
	2015		4,106
	2016		3,938
	2017		3,938
	2018		2,953


	Total	$	19,457




Schedule of future annual minimum sublease proceeds	'

	Future annual minimum sublease proceeds as of December 31, 2013, are as follows (in thousands):


	2014	$	832
	2015		252


	Total	$	1,084

Redeemable_Convertible_Preferr1

Redeemable Convertible Preferred Stock and Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2013
Redeemable Convertible Preferred Stock and Stockholders' Equity	'
Schedule of the Company's Preferred Stock prior to the closing of the initial public offering	'

	Prior to the closing of the initial public offering, at which time all shares of Preferred Stock were converted into shares of common stock, the Company's Preferred Stock consisted of the following (in thousands, except share and per share data):


		December 31,
		2012
	Series A Preferred Stock, $0.001 par value: 26,069,980 shares authorized, 6,410,976 shares issued and outstanding at December 31, 2012, at redemption value	$	66,665
	Series B Preferred Stock, $0.001 par value: 16,944,378 shares authorized, 4,204,185 shares issued and outstanding at December 31, 2012, at redemption value		67,044
	Series C Preferred Stock, $0.001 par value: 11,923,077 shares authorized, 2,978,062 shares issued, and outstanding at December 31, 2012, at redemption value		59,909
	Series C-1 Preferred Stock, $0.001 par value: 2,014,652 shares authorized, 457,875 issued, and outstanding at December 31, 2012, at redemption value		9,387
	Series D Preferred Stock, $0.001 par value: 955,414 shares authorized, 234,940 shares issued, and outstanding at December 31, 2012, at redemption value		4,325
	Series D-1 Preferred Stock, $0.001 par value: 2,802,548 shares authorized, 636,942 issued and outstanding at December 31, 2012, at redemption value		11,864
	Series E Preferred Stock, $0.001 par value: 3,662,422 shares authorized, 816,060 shares issued and outstanding at December 31, 2012, at redemption value		13,393
	Series F Preferred Stock, $0.001 par value: 9,704,756 shares authorized, 2,426,171 issued and outstanding at December 31, 2012, at redemption value		36,023


	Total redeemable convertible preferred stock	$	268,610




Schedule of number of shares of common stock reserved for future issuance	'

	At December 31, 2013, the Company has reserved for future issuance the following number of shares of common stock (in thousands):


		December 31,
		2013
	Outstanding stock options to purchase common stock		3,942
	Shares available for future issuance under stock option plan		956
	Warrants to purchase common stock		980
	Additional shares reserved for unissued, but designated, Preferred Stock		25,000


	Total shares of authorized common stock reserved for future issuance		30,878

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
Stock-Based Compensation (Tables)	Dec. 31, 2013
Stock-Based Compensation	'
Schedule of total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations and comprehensive (loss) income	'

	Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations and comprehensive (loss) income is as follows (in thousands):


		Year Ended December 31,
		2013			2012			2011
	Research and development	$	659		$	514		$	686
	General and administrative		1,537			692			741


		$	2,196		$	1,206		$	1,427




Schedule of weighted-average assumptions used for estimating fair value of each option issued to employees on the date of grant using the Black-Scholes option pricing model	'

	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:


		Year Ended December 31,
		2013			2012			2011
	Expected volatility		71.5	%		69	%		66	%
	Expected term (in years)		6			6			6
	Risk-free interest rate		1.85	%		0.9	%		1.1	%
	Expected dividend yield		—	%		—	%		—	%
Stock option plans	'
Stock-Based Compensation	'
Summary of the stock option activity	'

	The following table summarizes the stock option activity under the Company's stock option plans during the year ended December 31, 2013 (in thousands, except per share amounts and years):


		Number			Weighted-			Weighted-			Aggregate
		of Grants			Average			Average			Intrinsic
					Exercise			Contractual			Value(1)
					Price Per			Life (in years)
					Share
	Outstanding at December 31, 2012		3,730		$	4.16			6.62		$	13,946
	Granted		553		$	23.88
	Exercised		(293	)	$	2.24
	Canceled or forfeited		(48	)	$	4.39


	Outstanding at December 31, 2013		3,942		$	7.05			6.43		$	128,334
	Exercisable at December 31, 2013		2,544		$	4.07			5.13		$	90,401




	Vested and expected to vest at December 31, 2013(2)		3,860		$	6.89			6.37		$	126,258






	-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 December 31, 2013 and 2012.

	-2
	This represents the number of vested options at December 31, 2013, plus the number of unvested options expected to vest at December 31, 2013, based on the unvested options outstanding at December 31, 2013, adjusted for the estimated forfeiture rate.

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2013
Income Taxes	'
Schedule of significant components of the Company's deferred tax assets	'

	The significant components of the Company's deferred tax assets are comprised of the following (in thousands):


		Year Ended December 31,
		2013			2012
	Deferred tax assets:
	U.S. and state net operating loss carryforwards	$	51,886		$	35,584
	Research and development credits		5,519			5,384
	Deferred revenue		3,005			21,882
	Accruals and other temporary differences		5,675			5,333


	Total deferred tax assets		66,085			68,183
	Less valuation allowance		(66,085	)		(68,183	)


	Net deferred tax assets	$	—		$	—




Schedule of reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements	'

	A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:


		Year Ended December 31,
		2013			2012			2011
	Federal income tax expense at statutory rate		34	%		34	%		34	%
	State income tax, net of federal benefit		(1.5	)%		4.2	%		5	%
	Permanent differences		(43.1	)%		(3.4	)%		1.5	%
	Research and development credit		0.7	%		—	%		(1.0	)%
	Other		0	%		(0.4	)%		—	%
	Change in valuation allowance		9.9	%		(34.4	)%		(39.5	)%




	Effective income tax rate		0	%		0	%		0	%

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	12 Months Ended
Related Party Transactions (Tables)	Dec. 31, 2013
Related Party Transactions	'
Schedule of revenues from related party	'

	The Company recognized revenue from Celgene during the years ended December 31, 2013, 2012 and 2011 as follows (in thousands):


		Year Ended December 31,
		2013		2012		2011
	License and milestone	$	19,626	$	2,035	$	63,607
	Cost sharing, net		12,658		2,879		(121	)


		$	32,284	$	4,914	$	63,486

Quarterly_Financial_Data_unaud1

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
Quarterly Financial Data (unaudited) (Tables)	Dec. 31, 2013
Quarterly Financial Data (unaudited)	'
Schedule of certain unaudited quarterly financial information	'






		For the Three Months Ended(1)
		March 31			June 30			September 30			December 31
		(in thousands except per share data)
	2013:00:00
	Total revenue	$	15,012		$	26,427		$	4,270		$	11,521
	Total costs and expenses		(11,876	)		(12,276	)		(11,154	)		(14,972	)
	Income (loss) from operations		3,136			14,151			(6,884	)		(3,451	)
	Net income (loss)		1,647			13,078			(18,513	)		(18,110	)
	Basic net income (loss) per share*	$	(0.24	)	$	0.3		$	(5.62	)	$	(0.64	)
	Diluted net income (loss) per share*	$	(0.24	)	$	0.28		$	(5.62	)	$	(0.64	)
	2012:00:00
	Total revenue	$	3,324		$	4,040		$	3,905		$	3,985
	Total costs and expenses		(10,257	)		(10,944	)		(10,763	)		(12,179	)
	Loss from operations		(6,933	)		(6,904	)		(6,858	)		(8,194	)
	Net loss		(7,588	)		(7,400	)		(7,215	)		(10,379	)
	Basic net loss per share*	$	(1.50	)	$	(5.93	)	$	(5.82	)	$	(7.10	)
	Diluted net loss per share*	$	(1.50	)	$	(5.93	)	$	(5.82	)	$	(7.10	)


	-1
	The amounts were computed independently for each quarter, and the sum of the quarters may not total the annual amounts.

	*
	Applicable to common stockholders

Nature_of_Business_Details

Nature of Business (Details)	12 Months Ended
	Dec. 31, 2013
	item
Nature of Business	'
Number of protein therapeutics internally discovered	3
Number of ongoing Phase 2 clinical trials	12

Summary_of_Significant_Account3

Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
	item
Segment Information	'
Number of operating segments	1
Concentrations of Credit Risk and Off-Balance Sheet Risk	'
Off-balance sheet risk, asset	$0
Off-balance sheet risk, liability	$0

Summary_of_Significant_Account4

Summary of Significant Accounting Policies (Details 2) (Recurring, USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Quoted Prices in Active Markets for Identical Items (Level 1)	'	'
Assets:	'	'
Total assets	$102,307	$37,760
Quoted Prices in Active Markets for Identical Items (Level 1) \| Money market funds	'	'
Assets:	'	'
Total assets	101,394	36,847
Quoted Prices in Active Markets for Identical Items (Level 1) \| Restricted cash	'	'
Assets:	'	'
Total assets	913	913
Significant Unobservable Inputs (Level 3)	'	'
Liabilities:	'	'
Total liabilities	30,753	6,651
Significant Unobservable Inputs (Level 3) \| Preferred stock warrants	'	'
Liabilities:	'	'
Total liabilities	'	1,422
Significant Unobservable Inputs (Level 3) \| Common stock warrants	'	'
Liabilities:	'	'
Total liabilities	30,753	5,229
Total	'	'
Assets:	'	'
Total assets	102,307	37,760
Liabilities:	'	'
Total liabilities	30,753	6,651
Total \| Preferred stock warrants	'	'
Liabilities:	'	'
Total liabilities	'	1,422
Total \| Common stock warrants	'	'
Liabilities:	'	'
Total liabilities	30,753	5,229
Total \| Money market funds	'	'
Assets:	'	'
Total assets	101,394	36,847
Total \| Restricted cash	'	'
Assets:	'	'
Total assets	$913	$913

Summary_of_Significant_Account5

Summary of Significant Accounting Policies (Details 3) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
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	$6,651	$4,393
Change in fair value	26,875	2,258
Exercises	-678	'
Repurchases	-83	'
Conversions	-2,012	'
Ending balance	30,753	6,651
Transfers within the hierarchy	$0	$0

Summary_of_Significant_Account6

Summary of Significant Accounting Policies (Details 4) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	item	Dec. 31, 2012	Dec. 31, 2011
Property and Equipment	'	'	'
Impairment losses	$0	$0	$0
Stock-Based Compensation	'	'	'
Number of stock-based compensation plans	2	'	'
Computer equipment and software	'	'	'
Property and Equipment	'	'	'
Estimated useful life	'3 years	'	'
Office and laboratory equipment	'	'	'
Property and Equipment	'	'	'
Estimated useful life	'3 years	'	'

Summary_of_Significant_Account7

Summary of Significant Accounting Policies (Details 5)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	item	Dec. 31, 2012	Dec. 31, 2011
Net Income (Loss) Per Share	'	'	'
Number of years potentially dilutive shares excluded from weighted-average common share calculation due to net losses	2	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net income (loss) per share (in shares)	17,949	23,028	874
Outstanding stock options	'	'	'
Net Income (Loss) Per Share	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net income (loss) per share (in shares)	3,709	3,730	'
Common stock warrants	'	'	'
Net Income (Loss) Per Share	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net income (loss) per share (in shares)	908	884	874
Preferred stock	'	'	'
Net Income (Loss) Per Share	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net income (loss) per share (in shares)	13,218	18,166	'
Preferred stock warrants	'	'	'
Net Income (Loss) Per Share	'	'	'
Anti-dilutive common stock equivalents excluded from the calculation of diluted net income (loss) per share (in shares)	114	248	'

Property_and_Equipment_Net_Det

Property and Equipment, Net (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Property and equipment, net	'	'	'
Total property and equipment	$23,390	$23,009	'
Accumulated depreciation and amortization	-19,685	-18,950	'
Property and equipment, net	3,705	4,059	'
Depreciation and amortization expense	915	1,293	3,134
Computer equipment and software	'	'	'
Property and equipment, net	'	'	'
Total property and equipment	921	919	'
Office equipment	'	'	'
Property and equipment, net	'	'	'
Total property and equipment	205	179	'
Laboratory equipment	'	'	'
Property and equipment, net	'	'	'
Total property and equipment	12,334	11,815	'
Leasehold improvements	'	'	'
Property and equipment, net	'	'	'
Total property and equipment	9,930	10,088	'
Construction in progress	'	'	'
Property and equipment, net	'	'	'
Total property and equipment	'	$8	'

Restricted_Cash_Details

Restricted Cash (Details) (Money market account, Letters of credit, USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Money market account \| Letters of credit	'	'
Restricted Cash	'	'
Letters of credit held in the form of a money market account as collateral for lease obligations and credit cards	$0.90	$0.90

Accrued_Expenses_Details

Accrued Expenses (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Accrued Expenses	'	'
Collaboration expense	'	$1,000
Research and development related	1,330	1,282
Employee compensation	2,930	2,448
Professional services	1,017	607
Other	1,650	816
Total accrued expenses	$6,927	$6,153

Warrants_Details

Warrants (Details) (USD $)

12 Months Ended

0 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 20, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Mar. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jul. 31, 2010

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Common Stock

Series A Preferred Stock

Warrants to purchase preferred stock

Warrant to purchase Series A preferred stock

Warrant to purchase Series B preferred stock

Warrant to purchase Series C-1 preferred stock

Warrant to purchase Series D-1 preferred stock

Warrants to purchase Common stock

Warrants to purchase Common stock expiring between June 10, 2020 to July 9, 2020

Warrants to purchase Common stock expiring between March 31, 2015 to December 31, 2017

Warrants to purchase Common stock having expiration date as of June 25, 2019

Warrants to purchase Common stock expiring March 18, 2020

Maximum

Series A Preferred Stock