Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2016

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

Summary of Option Activity

The following is a summary of option activity under the 2015 Plan and 2005 Plan for the three months ended March 31, 2016:

	2015 Plan				2005 Plan
	Stock Options		Weighted- Average Exercise Price		Stock Options		Weighted- Average Exercise Price
Outstanding at January 1, 2016		8,357,003	$	8.97		15,450,542	$	3.31
Granted		9,723,437	$	4.99		—	$	—
Exercised		—	$	—		(511,650)	$	1.72
Canceled		(123,526)	$	8.37		(136,875)	$	4.48
Outstanding at March 31, 2016		17,956,914	$	6.82		14,802,017	$	3.35
Shares exercisable at March 31, 2016		1,916,292	$	8.94		10,041,892	$	2.79
Shares available for grant at March 31, 2016		6,998,086

Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The fair value of stock options granted under the 2015 Plan and 2005 Plan was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended  March 31,
	2016			2015
Weighted-average Black-Scholes fair value of stock options granted	$	2.45		$	2.92
Risk-free interest rate		1.25%-1.70	%		1.19	%
Dividend yield		0	%		0	%
Volatility		57.86%-68.28	%		53.58%-53.89	%
Expected term (in years)		4.26-7.28			4.26
Expected forfeiture rate		0%-16.33	%		16.33	%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the three months ended March 31, 2016:

	Number of Shares		Per Share Weighted- Average Grant-Date Fair Value
Outstanding and Unvested at January 1, 2016		25,000	$	8.72
Restricted stock granted		45,000	$	4.99
Restricted stock vested		—	$	—
Restricted stock forfeited		(25,000)	$	8.72
Outstanding and Unvested at March 31, 2016		45,000	$	4.99

Stock-Based Compensation Expense

The Company recorded all stock-based compensation expense in the consolidated statements of operations as follows (in thousands):

	Three Months Ended March 31,
	2016		2015
Research and development	$	3,233	$	1,032
General and administrative		1,728		900
Total stock-based compensation expense	$	4,961	$	1,932

Employee Stock Purchase Plan [Member]

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions: 

	Three Months Ended  March 31,
	2016		2015
Range of Black-Scholes fair values of ESPP shares granted		$1.86-$3.38		$1.06-$2.24
Risk-free interest rate		0.22%-0.44%		0.05%-0.35	%
Dividend yield		0	%	0	%
Volatility		46.14%-86.75%		40.79%-64.24	%
Expected term (in years)		0.5-2.0		0.5-2.0
Expected forfeiture rate		5	%	5	%