8. Equity-Based Compensation

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

	Year Ended December 31,
	2017		2016		2015
	(In thousands)
Research and development	$	7,280	$	4,083	$	1,061
General and administrative		8,042		2,632		247
Total	$	15,322	$	6,715	$	1,308

Restricted Stock

Restricted stock is measured at the fair value of the underlying security. Prior to the IPO, the Company valued these awards by taking into consideration its most recently available valuation performed by management and the board of directors, considering the most recently available third-party valuations of the Company’s securities as well as additional qualitative factors. In the periods subsequent to the IPO, fair value was determined based on the quoted price of the Company’s common stock.

The following table summarizes the Company’s restricted stock activity, including converted Founder Stock, for the year ended December 31, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2016		1,361,855			$	0.81
Vested		(791,571	)			0.69
Forfeited		(90,462	)			1.34
Unvested restricted stock as of December 31, 2017		479,822			$	0.90

As of December 31, 2017, there was $2.5 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 1.0 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 $12.43 per option for options granted during the year ended December 31, 2017, $6.48 per option for options granted during the year ended December 31, 2016 and $4.24 per option for options granted during the year ended December 31, 2015. Key assumptions used to apply this pricing model were as follows:

	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate		2.0	%		1.3	%		1.5	%
Expected life of options	6.0 years			6.0 years			6.0 years
Expected volatility of underlying stock		93.9	%		88.0	%		82.6	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The following is a summary of stock option activity for the year ended December 31, 2017:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2016		3,040,214		$	8.35
Granted		2,691,157			16.26
Exercised		(141,759	)		8.16
Forfeited		(884,164	)		12.55
Outstanding at December 31, 2017		4,705,448			12.09		7.63		35,087
Exercisable at December 31, 2017		1,685,074			9.06		5.26		17,172

As of December 31, 2017, there was $27.2 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 3.1 years.