Condensed Consolidated Statements of Cash Flows (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Cash flows from operating activities:
Net loss	$ (23,903)	$ (18,382)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,639	2,222
Depreciation and amortization	823	997
Changes in operating assets and liabilities:
Collaboration receivable	116	(138)
Prepaid expenses and other current assets	(401)	(164)
Other assets	(131)	207
Accounts payable	74	(2,677)
Accrued contract research costs	360	436
Other accrued liabilities	(1,285)	(1,702)
Deferred collaboration revenue	(2,285)	(2,361)
Net cash used in operating activities	(24,993)	(21,562)
Cash flows from investing activities:
Purchases of marketable securities	(39,663)	(6,047)
Maturities of marketable securities	22,342
Purchases of property and equipment	(45)	(39)
Net cash used in investing activities	(17,366)	(6,086)
Cash flows from financing activities:
Proceeds from issuance of common stock and exercise of common stock options, net of transaction costs	35,208	26,768
Proceeds from term loans	2,000
Payment of term loans	(92)
Payment of capital lease obligations	(176)	(664)
Net cash provided by financing activities	36,940	26,104
Net decrease in cash and cash equivalents	(5,419)	(1,544)
Cash and cash equivalents at beginning of period	31,310	44,155
Cash and cash equivalents at end of period	25,891	42,611
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 747	$ 88