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


	Year Ended December 31,
	2021		2020		2019
	(In thousands)
Research and development	$	26,712	$	10,202	$	6,986
General and administrative		20,297		9,701		8,105
Total	$	47,009	$	19,903	$	15,091

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 (“RSAs”), 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 December 31, 2021, there were 2,241,278 shares available for future issuance under the 2015 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 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.

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

The following table summarizes the Company’s RSU activity for the year ended December 31, 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		451,272			73.81
Vested		(119,715	)		20.94
Cancelled		(72,467	)		45.14
Unvested restricted stock units as of December 31, 2021		453,026		$	71.03

In January 2020, the Company granted 181,020 RSUs to certain non-executive employees that included a performance condition in addition to a service condition. These RSUs would vest over a period of three years and were 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. During the year ended December 31, 2020, the Company achieved one of its development milestones and 58,870 of these RSUs vested. During the year ended December 31, 2021, 26,235 of these RSUs vested based on the satisfaction of a service condition, and 64,290 vested due to the achievement of additional development milestones. At December 31, 2021, none of these RSUs are unvested.

The weighted-average grant date fair value of RSUs granted for the years ended December 31, 2021, 2020 and 2019 was $73.81, $21.70 and $0. The total fair value of RSUs vested (measured on the date of vesting) for the years ended December 31, 2021 and 2020 was $14.1 million and $2.8 million, respectively. During the year ended December 31, 2019, RSAs that were granted prior to the Company's IPO vested with a total fair value (measured on the date of vesting) of $0.6 million.

As of December 31, 2021, there was $27.1 million of unrecognized equity-based compensation expense related to RSUs that are 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 $54.09 per option for options granted during the year ended December 31, 2021, $9.07 per option for options granted during the year ended December 31, 2020, and $9.21 per option for options granted during the year ended December 31, 2019. The total intrinsic value (the amount by which the fair market value exceeded the exercise price) of stock options exercised during the years ended December 31, 2021, 2020 and 2019 was $262.0 million, $20.3 million, and $2.3 million, respectively. Weighted average assumptions used to apply this pricing model were as follows:


	Year Ended December 31,
	2021			2020			2019
Risk-free interest rate		1.0	%		0.8	%		2.1	%
Expected life of options	6.0 years			6.0 years			6.0 years
Expected volatility of underlying stock		72.9	%		67.8	%		68.1	%
Expected dividend yield		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 year ended December 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		2,729,302			84.16
Exercised		(2,700,886	)		15.22
Forfeited		(700,700	)		30.74
Outstanding at December 31, 2021		6,305,156		$	43.57		7.38	$	492,968
Exercisable at December 31, 2021		2,227,760		$	16.37		5.43	$	226,941

As of December 31, 2021, there was $130.3 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 3.1 years.

In May 2016, the Company adopted the 2016 Employee Stock Purchase Plan (the “2016 Plan”). The 2016 Plan allows eligible employees to purchase shares of the Company’s common stock on the last day of each predetermined six-month offering period at 85% of the lower of the fair market value per share at the beginning or end of the applicable offering period. The 2016 Plan provides for six-month offering periods beginning in January and July of each year.

As of December 31, 2021, there were 1,297,202 shares available for future issuance under the 2016 Plan. The number of shares reserved for issuance under the 2016 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.

During the years ended December 31, 2021, 2020, and 2019, the Company issued 30,897, 101,911, and 90,581 shares of common stock under the 2016 Plan, respectively. The weighted-average purchase prices of shares issued under the 2016 Plan were $65.51, $15.28, and $12.05 per share for the years ended December 31, 2021, 2020, and 2019, respectively.

The fair value of the awards issued under the 2016 Plan to employees was estimated at the beginning of the offering period using a Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2021			2020			2019
Risk-free interest rate	0.05%-0.09%			0.17%-1.6%			2.1%-2.5%
Expected term (in years)	0.5 years			0.5 years			0.5 years
Expected volatility of underlying stock	77.5%-109.2%			53.4%-98.3%			62.9%-76.5%
Expected dividend yield		0.0	%		0.0	%		0.0	%