In September 2019, the Company’s board of directors adopted, with the approval of its stockholders, the 2019 Equity Incentive Plan (the “2019 Plan”) for the issuance of incentive stock options (“ISO”), non-qualified stock options (“NSO”), stock appreciation rights (“SARs”), restricted stock, other stock awards and performance cash awards, to employees, non-employee directors, and consultants. The 2019 Plan became effective concurrent with the Company’s initial public offering (“IPO”).

Awards granted under the 2019 Plan expire no later than 10 years from the date of grant. For ISO and NSO, the option price shall not be less than 100% of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms. As of December 31, 2022, there are 12,911,263 shares available for the Company to grant under the 2019 Plan.

In September 2016, the Company adopted the 2016 Equity Incentive Plan (the “2016 Plan”) for the issuance of ISO, NSO, SARs, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Company’s board of directors and approved by the stockholders.

Awards granted under the 2016 Plan expire no later than 10 years from the date of grant. For ISO and NSO, the option price shall not be less than 100% of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.

In conjunction with adopting the 2019 Plan, the Company discontinued the 2016 Plan with respect to the new equity awards.

In September 2019, the Company’s board of directors adopted, with the approval of its stockholders, the ESPP. The ESPP became effective on the completion of the Company’s IPO.

The ESPP initially authorized the issuance of 1,280,000 shares of the Company’s common stock under purchase rights granted to its employees or employees of any of the Company’s designated affiliates. The number of shares of the Company’s common stock reserved for issuance is subject to an automatic increase at each calendar year. Under the ESPP, the Company may specify offerings with durations of not more than 27 months and may specify shorter purchase periods within each offering. The ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their earnings, subject to any plan limitations. Unless otherwise determined by the Company’s board of directors, employees can purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first date of an offering or the purchase date. During the year ended December 31, 2022, 118,288 shares were issued under the ESPP.

Activity under the Company’s stock option plans is set forth below:


	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(Years)		(in thousands)
Outstanding at December 31, 2021		10,308,928		$	31.75		8.2
Granted		2,051,535		$	28.07
Exercised		(696,963	)	$	6.51
Forfeited		(1,059,133	)	$	41.67
Outstanding at December 31, 2022		10,604,367		$	31.70		7.6	$	52,307
Vested and expected to vest at December 31, 2022		10,604,367		$	31.70		7.6	$	52,307
Vested and exercisable at December 31, 2022		6,069,564		$	27.06		7.0	$	48,178

The aggregate intrinsic value of options exercised during the years ended December 31, 2022, 2021 and 2020 was $12.1 million $65.1 million and $53.1 million, respectively.

During the years ended December 31, 2022, 2021, and 2020, the estimated weighted-average grant date fair value of the options granted was $22.69, $47.62, and $25.49 per share, respectively.

As of December 31, 2022, the Company expects to recognize the remaining unamortized stock-based compensation expense of $127.4 million related to stock options, over an estimated weighted average period of 2.2 years.

The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2022	2021	2020
Expected term of options (in years)	5.3 – 6.1	5.3 – 6.1	5.0 – 6.1
Expected stock price volatility	101.4% – 111.2%	103.1% – 112.1%	88.8% – 108.6%
Risk-free interest rate	1.6% – 4.3%	0.6% – 1.3%	0.3% – 1.2%
Expected dividend yield	—	—	—

Expected Term—The expected term represents the period that the stock options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term) as the Company has limited historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock option grants.

Expected Volatility—Since inception the expected volatility was determined by examining the historical volatilities for industry peers and using an average of historical volatilities of the Company’s industry peers. Beginning the first quarter of 2022, the expected volatility is determined by using a blended approach of the Company and its industry peers’ historical volatilities.

Risk-Free Interest Rate—The Company based the risk-free interest rate over the expected term of the stock options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant.

Expected Dividend Rate—The expected dividend is zero as the Company has not paid nor does it anticipate paying any dividends on its profit interest units in the foreseeable future.

In June 2021, the Company initiated its first offering period under the ESPP. Each offering period is six months, which commences on the grant date on or after June 1 and December 1 of each year and ends on the purchase date on or before November 30 and May 31 of each year.

The fair value of employees' purchase rights under the ESPP was estimated on the date of grant using the Black-Scholes option-pricing model with the following assumptions:


	Year Ended December 31,
	2022	2021
Expected term of ESPP (in years)	0.5	0.5
Expected stock price volatility	59.0% – 86.0%	76.1% – 144.1%
Risk-free interest rate	0.1% – 4.5%	0.04% – 0.1%
Expected dividend yield	—	—

The expected term of employees’ purchase rights is equal to the purchase period. The expected volatility was determined based on the Company's historical volatility. The risk-free interest rate is based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant over the expected term of the employees’ purchase rights. The expected dividend is zero as the Company has not paid nor does it anticipate paying any dividends on its profit interest units in the foreseeable future. Based on the Black-Scholes option-pricing model, the estimated weighted-average grant date fair value of the employees' purchase rights granted for the years ended December 31, 2022 and 2021 was $9.09 and $19.85 per share, respectively.

The Company’s RSAs and RSUs were summarized as follows:


	Shares			Weighted Average Grant Date Fair Value Per Share
	RSU			RSU
Unvested as of December 31, 2021		1,271,334		$	59.93
Granted		2,097,128		$	27.87
Vested		(349,496	)	$	58.26
Forfeited		(349,788	)	$	40.86
Unvested as of December 31, 2022		2,669,178		$	37.46

The unvested shares of RSUs have not been included in the shares issued and outstanding.

As of December 31, 2022, there was $77.0 million of total unrecognized compensation cost related to unvested restricted stock units, all of which is expected to be recognized over a remaining weighted-average period of 2.8 years.

Stock-based compensation is recognized on a straight-line basis over the requisite service period, which is generally the vesting period. The following table sets forth the total stock-based compensation expense for all awards granted to employees and the ESPP in the consolidated statements of operations:


	Year Ended December 31,
	2022		2021		2020
	(in thousands)
Research and development	$	53,153	$	42,554	$	13,663
Selling, general and administrative	$	48,929	$	41,230		13,937
Total stock-based compensation	$	102,082	$	83,784	$	27,600