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, 2021, there were 2,890,540 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,
	2021		2020
	(In thousands)
Research and development	$	3,491	$	2,160
General and administrative		2,933		1,997
Total	$	6,424	$	4,157

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, 2021:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2020		193,936		$	23.98
Granted		259,839			57.71
Vested		-			-
Cancelled		(11,035	)		27.59
Unvested restricted stock as of March 31, 2021		442,740		$	43.69

In March 2021, the Company granted 259,839 RSUs with a service condition to executive and non-executive employees as part of their annual grant, which vest over a period of four years. The weighted average grant date fair value of these RSUs was $57.71. The vesting start date for these RSUs is January 1, 2021.

Included in the unvested restricted stock as of March 31, 2021 are 107,360 RSUs 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. The fair value of the RSUs at date of grant was $15.05. There has been no additional vesting of these shares in the three months ended March 31, 2021.

As of March 31, 2021, there was $17.8 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 3.6 years.

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $36.64 and $7.96 per option for those options granted during the three months ended March 31, 2021 and 2020, 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, 2021 and 2020 was $54.8 million and $0.3 million, respectively. Weighted average assumptions used to apply this pricing model were as follows:

	Three Months Ended March 31,
	2021	2020
Risk-free interest rate	0.9%	1.0%
Expected life of options	6.0 years	6.0 years
Expected volatility of underlying stock	72.0%	66.7%
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 the Company’s historical volatility and an average of the historical stock volatilities of several peer companies within the Company’s industry, 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, 2021:

	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, 2020		6,977,440		$	15.43
Granted		1,645,823			57.94
Exercised		(1,014,569	)		13.15
Forfeited		(129,393	)		20.71
Outstanding at March 31, 2021		7,479,301		$	25.00		8.25	$	413,258
Exercisable at March 31, 2021		2,734,966

As of March 31, 2021, there was $86.9 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 3.2 years.