Common Stock and Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2015

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of December 31, 2015:



Common stock options outstanding	5,125,667
Common stock available for future grant under the 2012 Plan	1,549,163
Common stock available for future grant under the Inducement Plan	958,052
Employee Stock Purchase Plan	1,260,136
Total common shares reserved for future issuance	8,893,018

Stock Option Activity

The following table summarizes our stock option activity under all equity incentive plans for the year ended December 31, 2015 (shares and aggregate intrinsic value in thousands):



	Number of options		Weighted average exercise price		Weighted average remaining contractual term	Aggregate intrinsic value
Options outstanding at December 31, 2014	6,643		$	6.95
Granted	1,915		$	10.98
Exercised	(2,299	)	$	2.95
Canceled/forfeited/expired	(1,133	)	$	13.13
Options outstanding at December 31, 2015	5,126		$	8.91	7.6	$	8,461
Vested or expected to vest at December 31, 2015	4,911		$	8.83	7.6	$	8,319
Exercisable at December 31, 2015	1,963		$	6.37	5.3	$	5,816

Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan

The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2012 Plan and Inducement Plan and the shares purchasable under our 2012 Employee Stock Purchase Plan during the periods presented:



	Year ended December 31,
	2015		2014		2013
Stock options
Risk-free interest rate	1.8	%	1.8	%	1.9	%
Volatility	78.9	%	74.3	%	72.4	%
Dividend yield	—		—		—
Expected term (years)	6.1		6.1		6.1
Performance stock options
Risk-free interest rate	1.8	%	2.1	%	—	%
Volatility	76.7	%	69.6	%	—	%
Dividend yield	—		—		—
Expected term (years)	6.0		6.3		0.0
Employee stock purchase plan shares
Risk-free interest rate	0.1	%	0.1	%	0.1	%
Volatility	76.1	%	69.8	%	58.8	%
Dividend yield	—		—		—
Expected term (years)	0.5		0.5		0.5

Stock-Based Compensation

The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands):



	Year ended December 31,
	2015		2014		2013
Research and development	$	8,075	$	3,939	$	2,246
General and administrative	7,293		3,100		1,176
Total	$	15,368	$	7,039	$	3,422