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Stock-Based Compensation
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-based compensation
Stock-based compensation expense
The following table reflects the components of stock-based compensation expense recognized in our Consolidated Statements of Operations (in millions):
Year Ended December 31,
202320222021
Research and development$35 $33 $29 
General and administrative38 32 26 
Total stock-based compensation$73 $65 $55 
As of December 31, 2023, unrecognized compensation costs related to non-vested stock option awards and RSUs totaled $73 million and $35 million, respectively, and is expected to be recognized over a weighted average period of 2.2 years and 2.4 years, respectively.
Stock Plans
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.
2020 Stock Plan
In January 2020, we adopted the 2020 Inducement Plan (the "2020 Stock Plan") for the award of stock options and other equity-based awards as an inducement to eligible individuals to enter into employment with us. As of December 31, 2023, there were 3.2 million shares available for grant under this plan.
2015 Stock Plan and 2018 Equity Incentive Plan
In 2015, we adopted the 2015 Stock Plan, which in March 2018, we replaced with the 2018 Equity Incentive Plan (the "2018 Equity Incentive Plan"). Any outstanding awards under the 2015 Stock Plan that subsequently expire, lapse unexercised or are forfeited to us are added to the shares reserved for issuance under the 2018 Equity Incentive 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, 2023, there were 3.8 million shares available for grant under this plan. On January 1, 2024, the number of shares available for issuance under the 2018 Equity Incentive Plan automatically increased by 3.0 million.
Employee Stock Purchase Plan
In March 2018, we adopted the 2018 Employee Stock Purchase Plan (the "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 based on the stock price on the first day of the offering period. The 2018 ESPP is intended to constitute an “employee stock purchase plan” under IRC Section 423(b). The 2018 ESPP may be terminated by our board of directors at any time. 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, 2023, there were 2.4 million shares available for purchase under this plan. On January 1, 2024, the number of shares available for purchase under this Plan automatically increased by 0.8 million shares.
Restricted Stock Units
We grant RSUs to our employees and directors under the 2018 Equity Incentive 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, 2023:
Total Restricted Stock Units
(in millions) 		Weighted
Average
Grant Date Fair Value Per Share
Nonvested at December 31, 20221.3$31.09 
RSUs granted1.1$21.02 
RSUs vested(0.7)$28.93 
RSUs forfeited or canceled(0.2)$25.70 
Nonvested at December 31, 20231.5$25.43 
The total fair value of shares at vesting date during the years ended December 31, 2023, 2022 and 2021 was $13 million, $12 million and $12 million, respectively.
Stock Options
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, 2023:
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, 202212.0$21.77 
Options granted2.9$21.21 
Options exercised(0.3)$10.08 
Options forfeited or canceled(1.1)$29.26 
Outstanding at December 31, 202313.5$21.31 7.05$39 
Options vested and expected to vest as of December 31, 202313.5$21.31 7.05$39 
Options exercisable as of December 31, 20239.0$19.41 6.38$38 
During the years ended December 31, 2023, 2022 and 2021, the intrinsic value of shares exercised was $3 million, $26 million, and $17 million, respectively.
Valuation Assumptions for Stock Options and Employee Stock Purchase Plan
We utilize the Black-Scholes pricing model to estimate the fair value of stock options and shares issued under our 2018 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 Options202320222021
Weighted average closing price of our common stock on grant date$21.21 $30.37 $33.03 
Risk-free interest rate
3.6% - 4.6%
2.4% - 4.0%
1.0% - 1.4%
Expected term (in years)6.026.026.02
Volatility
77.7% - 79.1%
76.5% - 79.3%
75.3% - 77.6%
Dividend yield0%0%0%
Weighted average fair value of stock options granted$14.81 $20.75 $22.05 
Year Ended December 31,
ESPP202320222021
Risk-free interest rate
4.3% - 5.4%
1.6% - 4.7%
—% - 0.6%
Expected term (in years)
0.5 - 2.0
0.5-2.0
0.5-2.0
Volatility
61.1% - 86.9%
68.9% - 82.5%
61.2% - 95.7%
Dividend yield0%0%0%
Weighted average closing price of our common stock on grant date — Our awards are valued based on 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 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 — Our estimate of expected volatility is based on the historical volatility of our common stock price. Prior to 2023, due to our limited operating history and a lack of company specific historical and implied volatility data, our estimate of expected volatility included 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.