Condensed Statements of Operations and Comprehensive Loss (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]
Collaboration revenue	$ 568
Operating expenses:
Research and development	5,819	4,496
General and administrative	1,938	939
Total operating expenses	7,757	5,435
Loss from operations	(7,189)	(5,435)
Other income (expense):
Other income, net	51	2
Interest expense	(84)	(79)
Total other expense, net	(33)	(77)
Net loss and comprehensive loss	(7,222)	(5,512)
Cumulative preferred stock dividends	(519)	(893)
Net loss applicable to common stockholders	$ (7,741)	$ (6,405)
Net loss per share applicable to common stockholders-basic and diluted	$ (0.80)	$ (5.14)
Weighted-average number of common shares used in net loss per share applicable to common stockholders-basic and diluted	9,635	1,247