The Company grants awards to employees and nonemployees under a series of equity incentive plans, (collectively, the Stock Plans).

In May 2015, the Company adopted the 2015 Stock Plan, which was amended and restated in November 2015 (as amended from time to time, the 2015 Plan).

The terms of the 2015 Plan permitted option holders to exercise stock options before they vest, subject to certain limitations. Such unvested shares are subject to repurchase by the Company at the original exercise price in the

event the option holder’s service to the Company is terminated either voluntarily or involuntarily. As a result of early exercises under the 2015 Plan, approximately 9,946 shares and 165,133 shares had not vested and were subject to repurchase as of December 31, 2021 and 2020, respectively. The Company treats cash received from the exercise of unvested options as a refundable deposit and classifies such amounts as a liability in its consolidated balance sheets. As of December 31, 2021 and 2020, the Company included cash received for the early exercise of unvested options of $0.1 million and $0.7 million, respectively, allocated to other current liabilities. Amounts included in liabilities are transferred into common stock and additional paid-in capital as the shares vest, which is generally over a period of 48 months.

In March 2018, the Company adopted the 2018 Equity Incentive Plan (2018 Plan), which replaced the 2015 Plan upon completion of the IPO. 3,570,000 shares were reserved under the 2018 Plan plus 709,558 shares remaining available for issuance under the Company’s 2015 Plan and outstanding awards under its 2015 Plan that subsequently expire, lapse unexercised or are forfeited to or repurchased by the Company. In addition, the number of shares reserved for issuance under the Company's 2018 Plan will automatically increase on January 1 of each year by a number equal to the smallest of (i) 3,570,000 shares, (ii) 4% of the shares of common stock outstanding on the last business day of the prior fiscal year or (iii) the number of shares determined by the Company's board of directors. As of December 31, 2021, there were 2,226,271 shares available for grant under the 2018 plan.

In accordance with the provisions of the 2018 Plan, the number of shares available for issuance under the Plan automatically increased by 2,831,269 shares on January 1, 2022.

In January 2020, the Company’s Board of Directors adopted the 2020 Inducement Plan (2020 Plan), pursuant to which it reserved and authorized 3,000,000 shares of the Company’s common stock in order to award non-statutory stock options and other equity-based awards as a material inducement to eligible individuals to enter into employment with the Company. During the years ended December 31, 2021 and 2020, the Company’s Board of Directors authorized an increase of 5,000,000 shares and 1,000,000 shares, respectively, reserved for issuance under the 2020 Plan. As of December 31, 2021, 2,288,000 shares have been issued, and there were 3,743,219 shares available for grant under the 2020 Plan.

The following table, which includes options granted under the Company’s Stock Plans, summarizes option activity:


	Shares Subject to Outstanding Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2020		9,892,697		$	13.88
Options granted		3,758,769		$	33.03
Options exercised		(680,755	)	$	13.68
Options forfeited or canceled		(950,799	)	$	19.08
Outstanding at December 31, 2021		12,019,912		$	19.46		8.02	$	253,620
Options vested and expected to vest as of December 31, 2021		12,019,912		$	19.46		8.02	$	253,620
Options exercisable as of December 31, 2021		5,158,246		$	14.55		7.15	$	133,742

The weighted-average grant date fair value of the stock options granted was $22.05 per share for 2021, $11.57 per share for 2020, and $6.33 per share for 2019. During the years ended December 31, 2021, 2020 and 2019, the intrinsic value of shares exercised was $17.4 million, $6.8 million, and $0.2 million, respectively, and the fair value of shares vested during the same period was $52.4 million, $16.8 million, and $7.8 million, respectively.

Restricted Stock Units

The Company grants restricted stock units (RSUs) to its employees and directors under the 2018 Plan. The shares subject to the RSUs vest annually or quarterly over four years for employees and annually for directors.


	Total Restricted Stock Units			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2020		738,650		$	27.84
RSUs granted		932,362			34.05
RSUs vested		(382,183	)		31.60
RSUs forfeited or canceled		(212,741	)		26.06
Nonvested at December 31, 2021		1,076,088		$	32.23

The weighted-average grant date fair value of the RSUs granted was $34.05 per share for 2021 and $27.77 per share for 2020. The total grant date fair value of shares vested during the same periods was $12.1 million and $0.1 million, respectively. There were no RSUs granted or vested during the year ended December 31, 2019.

In March 2018, the Company adopted the 2018 Employee Stock Purchase Plan (2018 ESPP). The 2018 ESPP provides eligible employees with the opportunity to purchase shares of common stock through payroll deductions at a price equal to 85% of the lower of the fair market value per share on the first trading day of the applicable 24-month offering period or the fair market value per share on the applicable purchase date, provided that no more than 3,000 shares of common stock may be purchased by an employee on any purchase date. Also, the value of the shares purchased in any calendar year may not exceed $25,000. The 2018 ESPP is intended to constitute an “employee stock purchase plan” under Section 423(b) of the Internal Revenue Code of 1986, as amended. The 2018 ESPP may be terminated by the Company’s board of directors at any time. A total of 714,000 shares of common stock were initially reserved for issuance under the 2018 ESPP, and the number of shares reserved for issuance under the 2018 ESPP will automatically increase on January 1 of each year beginning on January 1, 2019 by a number of shares equal to the least of (i) 1% of the Company's outstanding shares of common stock on the last day of the prior fiscal year, (ii) 1,071,000 shares or (iii) a number of shares determined by the Company's board of directors.

As of December 31, 2021, there were 1,610,979 shares available for purchase under the 2018 ESPP. In accordance with the provisions of the 2018 ESPP, the number of shares available for purchase under the Plan automatically increased by 707,817 shares on January 1, 2022.

As of December 31, 2021, 2020 and 2019, 37,250, 31,986, and 372,774, respectively, of vested stock options and 1,291, 21,165, and 308,596, respectively, of unvested stock options were held by non-employees. The amount of stock-based compensation expense related to non-employees recognized in the consolidated financial statements for the years ended December 31, 2021, 2020 and 2019 was $0.2 million, $0.7 million and $0.9 million, respectively.

The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2021, 2020 and 2019, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):


	Year Ended December 31,
	2021		2020		2019
Research and development	$	28,302	$	11,195	$	4,152
General and administrative		26,226		10,630		4,829
Total stock-based compensation	$	54,528	$	21,825	$	8,981

As of December 31, 2021, unrecognized employee and nonemployee compensation costs related to non-vested stock option awards and RSUs totaled $102.9 million and $31.6 million, respectively, and is expected to be recognized over a weighted average period of 2.5 years and 2.7 years, respectively.

Prior to the Company’s IPO, the fair value of the shares of common stock underlying stock-based awards was determined by the board of directors, with input from management. Because there was no public market for the Company’s common stock, the board of directors determined the fair value of the common stock on the grant-date of the stock-based award by considering a number of objective and subjective factors, including enterprise valuations of the Company’s common stock performed by an unrelated third-party specialist, valuations of comparable companies, sales of the Company’s convertible preferred stock to unrelated third parties, operating and financial performance, the lack of liquidity of the Company’s capital stock, and general and industry-specific economic outlook. The board of directors intended all options granted to be exercisable at a price per share not less than the estimated per share fair value of common stock underlying those options on the date of grant.

Following the Company’s IPO, the market traded price of the shares of common stock underlying the stock-based awards is the fair value of the Company's stock as reported on the New York Stock Exchange on the grant date.

Company estimates the fair value of options and ESPP shares utilizing the Black-Scholes option pricing model, which is dependent upon several variables, such as expected term, volatility, risk-free interest rate, and expected dividends. Each of these inputs is subjective and generally requires significant judgment to determine. The following assumptions were used to calculate the fair value of stock-based compensation for the years ended December 31, 2021, 2020, and 2019:


	Stock Options
	Year Ended December 31,
	2021	2020	2019
Risk-free interest rate	1.0% - 1.4%	0.4% - 0.5%	1.6% - 2.3%
Expected term (in years)	6.02	6.02	6.02
Volatility	75.3% - 77.6%	76.5% - 78.5%	71.8% - 74.6%
Dividend yield	0%	0%	0%


	ESPP
	Year Ended December 31,
	2021	2020	2019
Risk-free interest rate	0.0% - 0.6%	0.1% - 0.2%	1.6% - 2.3%
Expected term (in years)	0.5-2.0	0.5-2.0	0.5-2.0
Volatility	61.2% - 95.7%	66.6% - 136.0%	64.8% - 77.1%
Dividend yield	0%	0%	0%

Expected Term — The Company has opted to use the “simplified method” for estimating the expected term of options, whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option (generally 10 years).

Expected Volatility — Due to the Company’s limited operating history and a lack of company specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility data was computed using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards.

Risk-Free Interest Rate — The risk-free rate assumption is based on the U.S. treasury yield in effect at the time of grant for instruments with maturities similar to the expected term of the Company’s stock options.

Expected Dividend — The Company has not issued any dividends in its history and does not expect to issue dividends over the life of the options and therefore has estimated the dividend yield to be zero.