STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income (loss)	$ 46	$ (35,670)	$ (38,379)
Adjustments to reconcile net income (loss) to net cash (used in) provided by operating activities:
Depreciation and amortization	56	47	320
Accretion and amortization on investments, net	2,098	2,889	1,322
Loss (gain) on retirement/sales of property and equipment, net		3	(831)
Loss on write-downs of property and equipment			200
Loss on sales of marketable securities	1
Stock-based compensation for services by non-employees	364	253	252
Stock-based compensation for employees and directors	8,397	7,658	4,435
Amortization related to 401(k) contributions	161	111	458
Unrealized (gain) loss on derivatives	(16)	(351)	316
Changes in assets and liabilities:
Interest and other receivables	(243)	(399)	188
Prepaid assets	89	(72)	1,081
Deposits and other assets		5	(4)
Accounts payable	(873)	(364)	(2,032)
Accrued compensation and benefits	(1,239)	580	(431)
Accrued collaboration charges	2,328
Accrued restructuring charges	52	(94)	(1,878)
Accrued liabilities	(417)	(246)	(1,697)
Deferred revenue	(35,000)	35,000
Net cash (used in) provided by operating activities	(24,196)	9,350	(36,680)
Cash flows from investing activities:
Restricted cash transfer	(1)	529	(1)
Purchases of property and equipment	(90)	(131)	(3)
Proceeds from sales of property and equipment			1,196
Purchases of marketable securities	(206,459)	(190,263)	(88,977)
Proceeds from sales/calls of marketable securities	4,242	10,549
Proceeds from maturities of marketable securities	202,381	101,412	108,839
Net cash provided by (used in) investing activities	73	(77,904)	21,054
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	2,575	98,360	6,553
Net cash provided by financing activities	2,575	98,360	6,553
Net (decrease) increase in cash and cash equivalents	(21,548)	29,806	(9,073)
Cash and cash equivalents, at beginning of year	42,796	12,990	22,063
Cash and cash equivalents, at end of year	$ 21,248	$ 42,796	$ 12,990