Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2015

Common Stock Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of June 30, 2015:

Common stock options outstanding	6,064,456
Common stock available for future grant	1,672,578
Employee Stock Purchase Plan	1,291,834
Total common shares reserved for future issuance	9,028,868

Stock Option Activity

The following table summarizes our stock option activity under all equity incentive plans for the six months ended June 30, 2015 (shares in thousands):

	Number of options			Weighted average exercise price
Options outstanding at December 31, 2014		6,643		$	6.95
Granted		1,141		$	12.50
Exercised		(1,206	)	$	1.48
Canceled/forfeited/expired		(514	)	$	11.32
Options outstanding at June 30, 2015		6,064		$	8.71

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 Equity Incentive Plan and the shares purchasable under our 2012 Employee Stock Purchase Plan during the periods presented:

	Three months ended June 30,						Six months ended June 30,
	2015			2014			2015			2014
Stock options
Risk-free interest rate		1.8	%		1.9	%		1.8	%		1.9	%
Volatility		79.1	%		72.4	%		78.8	%		73.1	%
Dividend yield		0	%		0	%		0	%		0	%
Expected term (years)		6.1			6.1			6.1			6.1
Performance stock options
Risk-free interest rate		—			2.0	%		1.8	%		2.1	%
Volatility		—			69.0	%		76.4	%		69.6	%
Dividend yield		—			0	%		0	%		0	%
Expected term (years)		—			6.3			6.0			6.3
Employee stock purchase plan shares
Risk-free interest rate		0.1	%		0.1	%		0.1	%		0.1	%
Volatility		76.9	%		71.5	%		74.9	%		68.4	%
Dividend yield		0	%		0	%		0	%		0	%
Expected term (years)		0.5			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):

	Three months ended June 30,				Six months ended June 30,
	2015		2014		2015		2014
Research and development	$	3,496	$	854	$	5,452	$	1,658
General and administrative		2,373		830		3,520		1,425
Total	$	5,869	$	1,684	$	8,972	$	3,083