The following table reflects the components of stock-based compensation expense recognized in our Consolidated Statements of Operations (in millions):


	Year Ended December 31,
	2022		2021		2020
Research and development	$	33	$	29	$	11
General and administrative		32		26		11
Total stock-based compensation	$	65	$	55	$	22

As of December 31, 2022, unrecognized compensation costs related to non-vested stock option awards and RSUs totaled $90 million and $37 million, respectively, and is expected to be recognized over a weighted average period of 2.2 years and 2.4 years, respectively.

We grant awards to employees and nonemployees under a series of equity incentive plans (collectively, the Stock Plans). Awards under our Stock Plans are made with newly issued shares reserved for this purpose.

In January 2020, we adopted the 2020 Inducement Plan (2020 Stock Plan). Under this plan, an initial 3.0 million shares of our common stock were authorized by the Board of Directors for the award of stock options and other equity-based awards as an inducement to eligible individuals to enter into employment with us. During the years ended December 31, 2021 and 2020, the Board of Directors authorized an increase of 5.0 million shares and 1.0 million shares, respectively. As of December 31, 2022, there were 3.2 million shares available for grant under this plan.

In March 2018, we adopted the 2018 Equity Incentive Plan (2018 Stock Plan), which replaced the 2015 Stock Plan. Under this plan, an initial 3.6 million shares were reserved together with 0.7 million shares that remained available for issuance at adoption under the 2015 Stock Plan. Any outstanding awards under the 2015 Stock Plan that subsequently expire, lapse unexercised or are forfeited to or repurchased by us are also added to the 2018 Stock Plan. The number of shares reserved for issuance will automatically increase on January 1 of each year by a number equal to or the smaller of (i) 3.6 million shares, (ii) 4% of the shares of common stock outstanding on the last business day of the prior fiscal year, or (iii) an amount as determined by the Board of Directors. As of December 31, 2022, there were 3.6 million shares available for grant under this plan. On January 1, 2023, the number of shares available for issuance under this Plan automatically increased by 2.9 million.

Our amended 2015 Stock Plan, which was in place prior to our public offering in March 2018, allowed option holders to exercise stock options prior to vesting, subject to certain limitations. Any exercised unvested shares were subject to repurchase by us at the original exercise price in the event the option holder’s service with the Company was terminated either voluntarily or involuntarily.

In March 2018, we 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 on the applicable purchase date. Employees are limited to a maximum purchase limit of 3,000 shares on each purchase date or $25,000 of shares purchased in a calendar year. 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 our board of directors at any time. Under this plan, an initial 0.7 million shares were reserved for issuance. The number of shares reserved for issuance will automatically increase on January 1 of each year by a number equal to or the smaller of (i) 1.1 million shares, (ii) 1% of the shares of common stock outstanding

on the last business day of the prior fiscal year, or (iii) an amount as determined by the Board of Directors. As of December 31, 2022, there were 2.0 million shares available for purchase under this plan. On January 1, 2023, the number of shares available for purchase under this Plan automatically increased by 0.7 million shares. The number of shares issued under the 2018 ESPP during the years ended December 31, 2022, 2021 and 2020 was 0.3 million, 0.2 million, and 0.2 million, respectively.

We grant restricted stock units (RSUs) to our employees and directors under the 2018 Plan. The RSUs vest annually or quarterly over four years for employees and annually for directors. The following table summarizes information regarding our RSUs for the year ended December 31, 2022:


	Total Restricted Stock Units (in millions)			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2021		1.1		$	32.23
RSUs granted		0.9			30.22
RSUs vested		(0.5	)		32.27
RSUs forfeited or canceled		(0.2	)		30.65
Nonvested at December 31, 2022		1.3		$	31.09

The total grant date fair value of shares vested during the years ended December 31, 2022, 2021 and 2020 was $15 million, $12 million and $0.1 million, respectively.

The exercise price of stock options is set at the closing price of our common stock on the grant date, and the related number of shares granted is fixed at that point in time. Awards expire 10 years from the date of grant. The following table summarizes information regarding our stock options for the year ended December 31, 2022:


	Shares Subject to Outstanding Options (in millions)			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2021		12.0		$	19.46
Options granted		2.3		$	30.37
Options exercised		(1.4	)	$	13.17
Options forfeited or canceled		(0.9	)	$	25.93
Outstanding at December 31, 2022		12.0		$	21.77		7.60	$	50
Options vested and expected to vest as of December 31, 2022		12.0		$	21.77		7.60	$	50
Options exercisable as of December 31, 2022		6.7		$	18.01		7.01	$	42

During the years ended December 31, 2022, 2021 and 2020, the intrinsic value of shares exercised was $26 million, $17 million, and $7 million, respectively, and the fair value of shares vested during the same period was $48 million, $40 million, and $17 million, respectively.

We utilize the Black-Scholes pricing model to estimate the fair value of stock options and shares issued under our ESPP. The following table summarizes the key assumptions used to calculate the fair value and the resulting weighted-average grant date fair value of stock options granted:


	Year Ended December 31,
Stock Options	2022		2021		2020
Weighted average closing price of our common stock on grant date	$	30.37	$	33.03	$	17.36
Risk-free interest rate	2.4% - 4.0%		1.0% - 1.4%		0.4% - 0.5%
Expected term (in years)	6.02		6.02		6.02
Volatility	76.5% - 79.3%		75.3% - 77.6%		76.5% - 78.5%
Dividend yield	0%		0%		0%
Weighted average fair value of stock options granted	$	20.75	$	22.05	$	11.57


	Year Ended December 31,
ESPP	2022	2021	2020
Risk-free interest rate	1.6% - 4.7%	0.0% - 0.6%	0.1% - 0.2%
Expected term (in years)	0.5 - 2.0	0.5-2.0	0.5-2.0
Volatility	68.9% - 82.5%	61.2% - 95.7%	66.6% - 136.0%
Dividend yield	0%	0%	0%

Closing price of our common stock on grant date — The exercise price of our stock options is set as the closing price of our common stock on the grant date.

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 our stock options.

Expected term — We have 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).

Volatility — Due to our limited operating history and a lack of company specific historical and implied volatility data, our estimate of expected volatility is based on the historical volatility of a group of similar publicly traded companies. 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.

Dividend yield — We have not issued any dividends in our history and do not expect to issue dividends over the life of the options.