Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2015

Schedule of Stock-Based Compensation Expense

The following stock-based compensation amounts were recognized for the periods indicated (in thousands):

	Three Months Ended June 30,						Six Months Ended June 30,
	2015			2014			2015			2014
Research and development		$	881		$	365		$	1,691		$	682
General and administrative			1,021			416			1,842			711
Total stock-based compensation expense		$	1,902		$	781		$	3,533		$	1,393

Schedule of Fair Value Option Award Estimated on the Date of Grant Using the Black-Scholes Option-Pricing Model

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:

	Six Months Ended June 30,
	2015	2014
Expected dividend yield	0%	0%
Expected volatility	74%	67 %
Risk-free interest rate	1.6% - 2.1%	2.1% - 2.3%
Expected term	6.25 years	6.25 years

Schedule of Stock Option Activity

The following table summarizes stock option activity under the Plan during the six months ended June 30, 2015:

	Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2014		3,572,116		$	11.40		7.3
Granted		137,388			32.17
Exercised		(205,617	)		2.26
Forfeited or expired		(19,425	)		18.25
Outstanding, June 30, 2015		3,484,462			12.72		7.1		$	87,994
June 30, 2015:
Exercisable		1,804,814			5.56		5.6			58,488
Vested and expected to vest		3,272,505			12.35		7.0			83,835