6.	Equity-Based Compensation

Equity-based compensation expense is classified in the consolidated statements of operations as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(In thousands)
Research and development	$	2,633	$	1,558	$	5,026	$	3,061
General and administrative		2,268		1,291		3,982		2,418
Total	$	4,901	$	2,849	$	9,008	$	5,479

Restricted Stock

The following table summarizes the Company’s restricted stock activity for the six months ended June 30, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of January 1, 2018		479,822		$	0.90
Granted		86,250			22.98
Vested		(298,229	)		0.74
Cancelled		(6,802	)		1.34
Unvested restricted stock as of June 30, 2018		261,041		$	8.37

As of June 30, 2018, there was $3.3 million of unrecognized equity-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.6 years.

Stock Options

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $16.64 per option and $17.94 per option for those options granted during the three and six months ended June 30, 2018, respectively, and $11.47 and $10.51 per option for those options granted during the three and six months ended June 30, 2017, respectively. Key assumptions used to apply this pricing model were as follows:

	Three Months Ended June 30,		Six Months Ended June 30,
	2018	2017	2018	2017
Risk-free interest rate	2.8%	1.5%	2.6%	1.9%
Expected life of options	5.5-6.25 years	6.0 years	5.5-6.25 years	6.0 years
Expected volatility of underlying stock	87.7%	94.0%	89.0%	92.7%
Expected dividend yield	0.0%	0.0%	0.0%	0.0%

The following is a summary of stock option activity for the six months ended June 30, 2018:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at January 1, 2018		4,705,448		$	12.09
Granted		998,286			24.09
Exercised		(777,484	)		9.36
Forfeited		(45,256	)		15.79
Outstanding at June 30, 2018		4,880,994		$	14.94		8.53	$	60,640
Exercisable at June 30, 2018		1,545,489		$	10.06		7.84	$	26,749

As of June 30, 2018, there was $37.9 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.9 years.