STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating activities
Net loss	$ (23,548)	$ (9,144)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	313	615
Accretion of investment discounts and amortization of investment premiums, net	314	233
Stock-based compensation expense	3,344	3,361
Changes in assets and liabilities:
Prepaids and other current and noncurrent assets	(2,501)	83
Accounts payable	1,611	(40)
Accrued compensation	(1,995)	(1,577)
Accrued restructuring charges	(445)	(398)
Accrued preclinical and clinical costs	485	(1,455)
Other accrued liabilities	1,002	(27)
Deferred revenue	(379)	(379)
Net cash used in operating activities	(21,799)	(8,728)
Investing activities
Purchases of investments	(34,242)	(32,579)
Proceeds from maturities of investments	42,662	28,975
Purchases of property and equipment	(168)	(18)
Net cash provided by (used in) investing activities	8,252	(3,622)
Financing activities
Net cash used in issuance of common stock and exercise of stock options	(1,576)	(501)
Net cash used in financing activities	(1,576)	(501)
Net decrease in cash and cash equivalents	(15,123)	(12,851)
Cash and cash equivalents at beginning of period	36,134	25,386
Cash and cash equivalents at end of period	$ 21,011	$ 12,535