Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2014

Common Stock Reserved for Future Issuance

The following shares of common stock are reserved for future issuance:

	September 30, 2014
Common stock options outstanding		6,271,214
Common stock available for future grant		1,398,649
Employee Stock Purchase Plan		840,071
Convertible note payable (Post-IPO GSK Note)		1,449,212
Total common shares reserved for future issuance		9,959,146

Stock Option Activity

The following table summarizes our stock option activity under all stock option plans for the nine months ended September 30, 2014 (in thousands):

	Number of options			Weighted average exercise price
Options outstanding at December 31, 2013		5,598		$	3.69
Granted		1,191		$	7.71
Exercised		(310	)	$	2.01
Canceled/forfeited/expired		(208	)	$	5.46
Options outstanding at September 30, 2014		6,271		$	4.47

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 September 30,						Nine months ended September 30,
	2014			2013			2014			2013
Stock options
Risk-free interest rate		2.0	%		1.7	%		1.9	%		1.4	%
Volatility		70.1	%		72.7	%		73.0	%		67.8	%
Dividend yield		0	%		0	%		0	%		0	%
Expected term (years)		6.1			6.1			6.1			6.1
Performance stock options
Risk-free interest rate		—			—			2.1	%		—
Volatility		—			—			69.6	%		—
Dividend yield		—			—			0	%		—
Expected term (years)		—			—			6.3			—
Employee stock purchase plan shares
Risk-free interest rate		0.07	%		0.10	%		0.07	%		0.11	%
Volatility		71.3	%		57.8	%		69.4	%		57.7	%
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 September 30,				Nine months ended September 30,
	2014		2013		2014		2013
Research and development	$	890	$	594	$	2,548	$	1,664
General and administrative		737		384		2,162		860
Total	$	1,627	$	978	$	4,710	$	2,524