Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended		134 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013
Operating activities
Net loss	$ (17,458)	$ (13,137)	$ (336,347)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash interest expense			525
Depreciation and amortization	439	420	12,207
Amortization of non-cash compensation			522
Stock-based compensation - employees	1,574	1,388	43,503
Stock-based compensation - non-employees			853
Stock-based license payments			1,220
Change in fair value of warrant liability	262	2,376	(1,291)
Loss on disposal of asset		27	388
Impairment of leasehold improvements			1,030
Non-cash charge for in-process research and development			418
Beneficial conversion feature related to bridge financing			135
Changes in operating assets and liabilities:
Receivable from GSK	1,917	742	(1,308)
Prepaid expenses and other current assets	517	3,114	(1,753)
Other non-current assets		267	(466)
Accounts payable and accrued expenses	(2,418)	(1,259)	6,427
Deferred reimbursements	1,267	(1,977)	31,685
Net cash used in operating activities	(13,900)	(8,039)	(242,252)
Investing activities
Sale and redemption of marketable securities	34,135	14,300	789,577
Purchases of marketable securities	(26,998)	(67,906)	(847,693)
Purchases of property and equipment	(372)	(3,082)	(18,585)
Net cash (used in)/provided by investing activities	6,765	(56,688)	(76,701)
Financing activities
Proceeds from the issuance of preferred stock, net of issuance costs			143,022
Proceeds from the issuance of common stock and warrants, net of issuance costs		62,063	193,441
Proceeds from the issuance of convertible notes			5,000
Payments of capital lease obligations			(5,587)
Payments of secured loan agreement	(100)	(313)	(4,156)
Proceeds from exercise of stock options		837	3,341
Proceeds from exercise of warrants (common and preferred)			264
Proceeds from capital asset financing arrangement			5,611
Proceeds from secured loan agreement		995	4,753
Net cash provided by/(used in) financing activities	(100)	63,582	345,689
Net (decrease)/increase in cash and cash equivalents	(7,235)	(1,145)	26,736
Cash and cash equivalents at beginning of period	33,971	25,668
Cash and cash equivalents at end of period	26,736	24,523	26,736
Supplemental disclosures of cash flow information
Cash paid during the period for interest	7	20	2,124
Non-cash activities
Conversion of warrants to common stock			386
Conversion of notes payable to Series B redeemable convertible preferred stock			5,000
Conversion of preferred stock to common stock			148,951
Accretion of redeemable convertible preferred stock			802
Beneficial conversion feature related to the issuance of Series C redeemable convertible preferred stock			$ 19,424