Stock Based Compensation (Tables)

6 Months Ended

Jun. 30, 2014

Summary of Option Activities

The following table summarizes option activity under our 2013 Equity Incentive Plan and related information during the six months ended June 30, 2014:

				Shares
				Subject to			Weighted-
	Shares Available			Outstanding			Average Exercise
	for Grant			Options			Price Per Share
Balance at December 31, 2013		81,948			3,708,773		$	9.43
Options authorized		2,045,785			—			—
Options granted		(1,003,013	)		1,003,013			25.47
Options exercised		—			(433,146	)		6.34
Options canceled		52,470			(52,470	)		11.77
Balance at June 30, 2014		1,177,190			4,226,170		$	13.53

Fair Values of Employee Stock Options

The estimated grant date fair values of the employee stock options were calculated using the Black Scholes valuation model, based on the following assumptions:

	Three Months Ended June 30,						Six Months Ended June 30,
	2014			2013			2014			2013
Risk-free interest rate		1.89	%		1.20	%		1.89	%		1.10	%
Expected life	5.8				6.0		5.9				6.0
Expected Volatility		80	%		79	%		80	%		79	%
Dividend yield		—			—			—			—

Classification of Stock-Based Compensation Expense

The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands):  

	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Research and development	$	1,283	$	609	$	2,330	$	945
General and administrative		1,140		601		2,396		978
Total stock-based compensation	$	2,423	$	1,210	$	4,726	$	1,923