Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Collaboration revenue	$ 67	$ 539	$ 425	$ 1,868
Operating expenses:
Research and development	6,745	8,872	18,252	21,445
General and administrative	2,681	2,269	7,531	6,259
Total operating expenses	9,426	11,141	25,783	27,704
Loss from operations	(9,359)	(10,602)	(25,358)	(25,836)
Other income (expense):
Other income, net	73	2	3,207	56
Interest expense	(407)	(74)	(972)	(237)
Total other income (expense), net	(334)	(72)	2,235	(181)
Net loss and comprehensive loss	(9,693)	(10,674)	(23,123)	(26,017)
Cumulative preferred stock dividends	0	0	0	(519)
Net loss applicable to common stockholders	$ (9,693)	$ (10,674)	$ (23,123)	$ (26,536)
Net loss per share applicable to common stockholders—basic and diluted (in USD per share)	$ (0.50)	$ (0.66)	$ (1.23)	$ (1,901.68)
Weighted-average number of common shares used in net loss per share applicable to common stockholders—basic and diluted	19,345	16,098	18,806	14