Stock Based Compensation (Tables)

9 Months Ended

Sep. 30, 2014

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activities

The following table summarizes option activity under our 2013 Equity Incentive Plan and related information during the nine months ended September 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		(1,114,013	)		1,114,013			25.50
Options exercised		—			(517,481	)		6.41
Options canceled		145,410			(145,410	)		18.26
Balance at September 30, 2014		1,159,130			4,159,895		$	13.80

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 September 30,						Nine Months Ended September 30,
	2014			2013			2014			2013
Risk-free interest rate		1.89	%		1.80	%		1.89	%		1.30	%
Expected life	5.8				6.0		5.9				6.0
Expected Volatility		79	%		80	%		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 September 30,				Nine Months Ended September 30,
	2014		2013		2014		2013
Research and development	$	953	$	787	$	3,321	$	1,732
General and administrative		1,197		747		3,555		1,725
Total stock-based compensation	$	2,150	$	1,534	$	6,876	$	3,457