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Stock-Based Compensation​ (Tables)
12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Schedule of Share-Based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions The following table sets forth the key assumptions and fair value results for each award granted in the Company’s six-month offering period that started on December 1, 2023:
InputDecember 1, 2023
Stock price$6.30 
Risk-free interest rate5.26 %
Term (in years)0.50
Volatility97.50 %
Dividend yield0.00 %
Grant date fair value per share$2.62 
Share-based Payment Arrangement, Option, Activity
A summary of the Company’s stock option activity is as follows:
Number of
Shares
Weighted
Average
Exercise
Price
Weighted
Average
Remaining
Contractual
Life (Years)
Aggregate
Intrinsic
Value
(In millions)
Outstanding as of January 1, 2023
5,335,974 $0.12 7.66$9.4 
Exercised(2,027,221)0.12 7.8 
Expired/forfeited(134,639)0.12 
Outstanding as of December 31, 2023
3,174,114 0.12 6.6619.1 
Exercisable as of December 31, 2023
1,346,269 $0.13 6.75$8.1 
Vested and expected to vest as of December 31, 2023
3,174,114 0.12 6.6619.1 
Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions
The assumptions used in the Black-Scholes option pricing model to estimate the fair value of the stock options granted during the year ended December 31, 2021 are provided in the following table:
December 31, 2021
Risk-free interest rate:
Employee stock options0.62 %
Non-employee stock options1.08 %
Expected term (in years):
Employee stock options6.32
Non-employee stock options10.00
Expected volatility:
Employee stock options87.94 %
Non-employee stock options88.03 %
Dividend yield:
Employee stock options0.00 %
Non-employee stock options0.00 %
Grant date fair value per share:
Employee stock options$13.65 
Non-employee stock options$13.68 
Share-based Payment Arrangement, Restricted Stock Unit, Activity
A summary of the Company’s RSU activity is as follows:
Number of
Shares
Weighted
Average
Grant Price
Outstanding as of January 1, 2023
43,146,632 $5.72 
Granted11,864,674 3.36 
Vested(7,555,232)3.95 
Forfeited (15,933,591)6.26 
Outstanding as of December 31, 2023
31,522,483 4.99 
Schedule of Share-Based Payment Award, Equity Instruments Other Than Options, Valuation Assumptions
The following assumptions were used to estimate the fair value, using the Monte Carlo simulation, of the market award grant:

September 16, 2021
Stock price$9.92 
Term (in years)7
Volatility55.00 %
Risk-free interest rate1.13 %
Dividend yield0.00 %
Share-based Payment Arrangement, Expensed and Capitalized, Amount
The following table presents stock-based compensation expense included in each respective expense category in the consolidated statements of operations (in millions):
Year Ended December 31,
202320222021
Research and development$28.9 $26.1 $3.7 
General and administrative16.3 76.7 119.9 
Total stock-based compensation expense$45.2 $102.8 $123.6 
Share-based Payment Arrangement, Warrant Activity
A summary of the Company’s warrant activity is as follows:
Number of
Shares
Weighted
Average
Exercise
Price
Weighted
Average
Remaining
Contractual
Life (Years)
Aggregate
Intrinsic
Value
(In millions)
Outstanding as of January 1, 2023
8,736,599 $0.01 7.84$16.3 
Issued15,000,000 0.01 
Exercised(2,948,352)0.01 17.0 
Outstanding as of December 31, 2023
20,788,247 0.01 4.48127.4 
Vested as of December 31, 2023
2,839,893 $0.01 2.17$17.4