Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2016

Stock-based compensation expense was allocated to research and development and general and administrative expense as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
Research and development	$	5,059	$	4,434	$	9,556	$	8,656
General and administrative		3,345		2,508		6,188		4,732
Total stock-based compensation expense	$	8,404	$	6,942	$	15,744	$	13,388

The assumptions used to estimate the fair value of stock options granted and ESPPs using the Black-Scholes option valuation model were as follows:

	Three Months Ended June 30,						Six Months Ended June 30,
	2016			2015			2016			2015
Stock Options
Expected term (in years)		5.3			5.2			5.3			5.2
Expected volatility		72	%		70	%		70	%		70	%
Risk-free interest rate		1.3			1.7			1.4			1.7
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	11.60		$	12.27		$	11.48		$	16.68

ESPPs
Expected term (in years)	0.5 - 2.0			0.4 - 2.0			0.5 - 2.0			0.4 - 2.0
Expected volatility	61.9 - 80.7%			58.5 - 69.0%			61.9 - 80.7%			58.5 - 69.0%
Risk-free interest rate	0.2 - 0.9%			0.1 - 0.6%			0.2 - 0.9%			0.1 - 0.6%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	10.06		$	13.06		$	10.98		$	13.17