STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended			100 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011
Cash flows from operating activities:
Net loss	$ (63,778)	$ (32,670)	$ (7,649)	$ (187,592)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	268	188	330	1,916
Stock-based compensation	2,759	3,637	2,181	15,301
Change in fair value of warrants	(7,583)	8,889	(4,461)	(4,721)
Loss on disposal of fixed assets				9
Common stock issued in exchange for in-process research and development	17,457			17,457
(Increase) decrease in:
Collaboration receivable	(79)			(79)
Prepaid expenses and other current assets	(889)	(70)	(27)	(1,313)
Other noncurrent assets	(407)	(122)	49	(771)
Deposits	(4)	(41)	41	(91)
Increase (decrease) in:
Accounts payable	696	(758)	(850)	1,727
Accrued expenses	8,283	1,277	(1,876)	10,821
Deferred revenue	4,333			4,333
Deferred rent	109	(24)	(32)	195
Net cash used in operating activities	(38,835)	(19,694)	(12,294)	(142,808)
Cash flows from investing activities:
Purchases of property and equipment	(1,156)	(186)	(11)	(3,067)
Proceeds from sale of property and equipment				1
Net cash used in investing activities	(1,156)	(186)	(11)	(3,066)
Cash flows from financing activities:
Stockholders' capital contribution				500
Proceeds from exercise of stock options	980	225	73	1,343
Payments to employees for repurchase of restricted common stock	(274)	(1,668)	(380)	(2,321)
Proceeds from exercise of warrants	12,399	72	279	12,749
Proceeds from issuance of common stock and warrants, net	71,207	32,804	49,793	221,556
Proceeds from issuance of preferred stock, net				16,760
Net cash provided by financing activities	84,312	31,433	49,765	250,587
Net increase (decrease) in cash and cash equivalents	44,321	11,553	37,460	104,713
Cash and cash equivalents, beginning of period	60,392	48,839	11,379
Cash and cash equivalents, end of period	104,713	60,392	48,839	104,713
Supplementary disclosure of cash flow information:
Cash paid for interest
Cash paid for income taxes
Supplementary disclosure of noncash investing and financing activities:
Warrants issued to placement agents and investors			27,068	47,276
Exercise of liability-classified warrants to common shares	303	49		352
Convertible Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock				16,760
Warrant
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock	$ 9,067	$ 239		$ 9,324