Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 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 six months ended June 30, 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		10,104,437	$	5.04		—	$	—
Exercised		—	$	—		(620,100)	$	2.01
Canceled		(321,052)	$	7.29		(196,000)	$	4.69
Outstanding at June 30, 2016		18,140,388	$	6.81		14,634,442	$	3.34
Shares exercisable at June 30, 2016		1,991,605	$	8.97		10,469,067	$	2.81
Shares available for grant at June 30, 2016		12,814,612

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  June 30,		Six Months Ended  June 30,
	2016	2015	2016	2015
Weighted-average Black- Scholes fair value of stock options granted	$2.99	$4.42	$2.47	$4.42
Risk-free interest rate	1.07%-1.09%	1.37%-2.13%	1.07%-1.70%	1.19%-2.13%
Dividend yield	0%	0%	0%	0%
Volatility	58.10%-58.97%	54.18%-68.39%	57.86%-68.28%	53.58%-68.39%
Expected term (in years)	4.24-4.26	3.98-7.34	4.24-7.28	3.98-7.34
Expected forfeiture rate	10.31%	0%-16.33%	0%-16.33%	0%-16.33%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the six months ended June 30, 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 June 30, 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 June 30,				Six Months Ended June 30,
	2016		2015		2016		2015
Research and development	$	2,952	$	1,090	$	6,185	$	2,122
General and administrative		2,305		1,490		4,033		2,392
Total stock-based compensation expense	$	5,257	$	2,580	$	10,218	$	4,514

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 June 30,		Six Months Ended June 30,
	2016	2015	2016	2015
Range of Black-Scholes fair value of ESPP shares granted	$1.97-$3.88	$1.20-$2.24	$1.86-$3.88	$1.06-$2.24
Risk-free interest rate	0.32%-0.47%	0.07%-0.35%	0.22%-0.47%	0.05%-0.35%
Dividend yield	0%	0%	0%	0%
Volatility	43.03%-86.75%	40.79%-64.24%	43.03%-86.75%	40.79%-64.24%
Expected term (in years)	0.5-2.0	0.5-2.0	0.5-2.0	0.5-2.0
Expected forfeiture rate	5%	5%	5%	5%