In April 2016, the Company adopted the Amended and Restated 2015 Stock Option and Incentive Plan (the “2015 Plan”). The 2015 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock units (“RSUs”) and other stock-based awards. Recipients of incentive stock options and non-qualified stock options are eligible to purchase shares of the Company’s common stock at an exercise price equal to the fair value of such stock on the grant date. Stock options granted under the 2015 Plan generally vest 25% on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years, unless they contain specific performance-based vesting provisions. The maximum term of stock options granted under the 2015 Plan is ten years.

As of March 31, 2020, there were 2,556,282 shares available for future issuance. The number of shares reserved for issuance under the 2015 Plan shall be cumulatively increased by four percent of the number of shares of stock issued and outstanding on the immediately preceding December 31 or such lesser number of shares of stock as determined by the board of directors.

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

	Three Months Ended March 31,
	2020		2019
	(In thousands)
Research and development	$	2,160	$	1,783
General and administrative		1,997		2,809
Total	$	4,157	$	4,592

Restricted stock is measured at fair value based on the quoted price of the Company’s common stock.

The following table summarizes the Company’s restricted stock activity for the three months ended March 31, 2020:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2019		71,875		$	22.88
Granted		181,020			15.05
Vested		-			-
Cancelled		(4,410	)		15.05
Unvested restricted stock as of March 31, 2020		248,485		$	17.31

As of March 31, 2020, there was $2.6 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 2.6 years. As of March 31, 2020, 71,875 of the unvested restricted stock outstanding are performance-based RSUs that vest upon obtaining certain scientific, financial and regulatory milestones through 2020. These performance-based RSUs are not included in computing the diluted loss per share because the performance criteria had not been met as of the end of the reporting period.

In January 2020, the Company granted 181,020 RSUs to certain employees that include a performance condition in addition to a service condition. The RSUs vest over a period of three years and are subject to accelerated vesting based on the Company’s programs achieving certain development milestones before December 1, 2022. To date, the Company has not accelerated the vesting of the RSUs. The grant date fair value of the RSUs is $15.05.

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $7.96 and $8.94 per option for those options granted during the three months ended March 31, 2020 and 2019, respectively. The total intrinsic value (the amount by which the fair market value exceeded the exercise price) of stock options exercised during the three months ended March 31, 2020 and 2019 was $0.3 million and $0.1 million, respectively. Key assumptions used to apply this pricing model were as follows:

	Three Months Ended March 31,
	2020	2019
Risk-free interest rate	1.0%	2.6%
Expected life of options	6.0 years	6.0 years
Expected volatility of underlying stock	66.7%	69.2%
Expected dividend yield	0.0%	0.0%

Risk-free Interest Rate. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant with maturities approximately equal to the option’s expected term.

Expected Dividend Yield. The expected dividend yield assumption is based on the fact that the Company has never paid cash dividends and has no present intention to pay cash dividends.

Expected Volatility. The expected volatility was derived from a blend of average historical stock volatilities of several peer companies within the Company’s industry and the Company’s historical volatility, both over a period equivalent to the expected term of the stock option grants.

Expected Term. The expected term represents the period that stock option awards are expected to be outstanding. For option grants that are considered to be “plain vanilla,” the Company determines the expected term using the simplified method. The simplified method deems the term to be the average of the time-to-vesting and the contractual life of the options. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate the expected term.

The Company uses the market closing price of its common stock as reported on the Nasdaq Global Select Market to determine the fair value of the shares of common stock underlying stock options. The following is a summary of stock option activity for the three months ended March 31, 2020:

	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, 2019		5,365,971		$	15.67
Granted		2,103,480			13.31
Exercised		(53,579	)		6.28
Forfeited		(172,777	)		16.23
Outstanding at March 31, 2020		7,243,095		$	15.04		8.22	$	5,134
Exercisable at March 31, 2020		2,847,849		$	14.35		6.76	$	5,134

As of March 31, 2020, there was $39.3 million of unrecognized compensation cost related to stock options that have not yet vested. These costs are expected to be recognized over a weighted average remaining vesting period of 2.8 years.

Of the unvested stock options outstanding as of March 31, 2020, 213,750 are performance-based stock options that vest upon obtaining certain scientific, financial and regulatory milestones through 2020. At March 31, 2020, 173,750 performance-based options are not included in computing the diluted loss per share because the performance criteria had not been met as of the end of the reporting period.