Condensed Statements of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Collaboration and license revenue	$ 2,372	$ 3,108
Operating expenses:
Research and development	28,155	17,722
General and administrative	5,241	3,039
Total operating expenses	33,396	20,761
Loss from operations	(31,024)	(17,653)
Interest and other income (expense), net	298	(489)
Net loss	$ (30,726)	$ (18,142)
Net loss per share attributable to common stockholders:
Basic and diluted	$ (0.75)	$ (12.94)
Shares used to compute net loss per share attributable to common stockholders:
Basic and diluted	41,001,623	1,401,677