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 September 30, 2021, there were 3,787,307 shares available for future issuance under the 2015 Plan and the 2016 Employee Stock Purchase Plan. The number of shares reserved for issuance under the 2015 Plan shall be cumulatively increased by four percent of the number of shares of common stock issued and outstanding on the immediately preceding December 31 or such lesser number of shares of common stock as determined by the board of directors. The number of shares reserved for issuance

under the 2016 Employee Stock Purchase Plan shall be cumulatively increased by the lesser of a) one percent of the number of shares of common stock issued and outstanding on the immediately preceding December 31, b) 500,000 shares of common stock, or c) such lesser number of shares of common 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 September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
	(In thousands)
Research and development	$	9,017	$	2,775	$	18,643	$	7,325
General and administrative		6,393		2,625		13,805		6,996
Total	$	15,410	$	5,400	$	32,448	$	14,321

RSUs are 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 nine months ended September 30, 2021:


	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock units as of December 31, 2020		193,936		$	23.98
Granted		354,072			63.13
Vested		(50,946	)		17.80
Cancelled		(53,777	)		38.02
Unvested restricted stock units as of September 30, 2021		443,285		$	54.26

In March 2021, the Company granted 259,839 RSUs with a service condition to 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.

Shares vesting during the nine months ended September 30, 2021 included 26,235 shares that vested in June 2021 as a service condition was met and 21,430 shares that were accelerated in September 2021 based on the Company reaching a performance milestone. Included in the unvested restricted stock as of September 30, 2021 are 42,860 RSUs that include a performance condition in addition to a service condition. These RSUs would vest over a period of 1.2 years and were subject to accelerated vesting based on the Company’s programs achieving certain development milestones before December 1, 2022. In October 2021, these development milestones were achieved and, accordingly, the vesting of the RSUs was accelerated during the fourth quarter of 2021. The fair value of the RSUs at date of grant was $15.05.

As of September 30, 2021, there was $19.9 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 2.8 years.

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $105.17 and $50.73 per option for those options granted during the three and nine months ended September 30, 2021 and $13.38 and $8.55 per option for those options granted during the three and nine months ended September 30, 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 and nine months ended September 30, 2021 was $171.3 million and $255.8 million, respectively, and during the three and nine months ended September 30, 2020 was $0.9 million and $1.7 million, respectively. Weighted average assumptions used to apply this pricing model were as follows:


	Three Months Ended September 30,		Nine Months Ended September 30,
	2021	2020	2021	2020
Risk-free interest rate	1.0%	0.4%	0.9%	0.9%
Expected life of options	6.0 years	6.0 years	6.0 years	5.5-6.0 years
Expected volatility of underlying stock	75.2%	70.8%	72.7%	67.6%
Expected dividend yield	0.0%	0.0%	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 nine months ended September 30, 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		2,435,102			79.17
Exercised		(2,636,437	)		15.12
Forfeited		(559,241	)		29.80
Outstanding at September 30, 2021		6,216,864		$	39.23		8.01	$	601,927
Exercisable at September 30, 2021		1,977,984