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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Employee and Non-Employee Stock Option Activity | The following table summarizes employee and non-employee stock option activity for the year ended December 31, 2023:
| Shares Issuable Under Options |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
|  |  |  |  | (in years) |  | (in thousands)
Outstanding as of December 31, 2022 | 405,082 |  | $ | 113.75 |  |  | 6.72 |  | $ | 2 |
Granted | 8,776,245 |  | 9.67 |  |  |  |  |
Exercised | (46,246) |  | 8.22 |  |  |  |  |
Forfeited | (637,686) |  | 43.00 |  |  |  |  |
Outstanding as of December 31, 2023 | 8,497,395 |  | $ | 12.13 |  |  | 8.40 |  | $ | 98,928 |
Options vested and expected to vest as of December 31, 2023 | 8,497,395 |  | $ | 12.13 |  |  | 8.40 |  | $ | 98,928 |
Options exercisable as of December 31, 2023 | 1,065,700 |  | $ | 24.72 |  |  | 5.62 |  | $ | 13,328 |
Employee Restricted Stock Activity | The following table summarizes employee restricted stock activity for the year ended December 31, 2023:
| Shares |  | Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2022 | 5,660 |  | $ | 203.25 |
Granted | 153,865 |  | 18.17 |
Vested | — |  | — |
Forfeited | (5,660) |  | 203.25 |
Unvested restricted stock units as of December 31, 2023 | 153,865 |  | $ | 18.17 |
Stock-Based Compensation Expense | Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, and the 2016 ESPP for the years ended December 31, 2023, 2022, and 2021 was as follows (in thousands):
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
| Employees |  | Non- Employees |  | Employees |  | Non- Employees |  | Employees |  | Non- Employees
Research and development | $ | 2,910 |  |  | $ | 11,328 |  |  | $ | 2,591 |  |  | $ | — |  |  | $ | 2,723 |  |  | $ | — |
General and administrative | 11,327 |  |  | 109 |  |  | 4,520 |  |  | — |  |  | 5,315 |  |  | — |
Total stock-based compensation expense | $ | 14,237 |  |  | $ | 11,437 |  |  | $ | 7,111 |  |  | $ | — |  |  | $ | 8,038 |  |  | $ | — |
Weighted-Average Assumptions Used in Calculating Fair Value of Awards | The following table summarizes the weighted-average assumptions used in calculating the fair value of the awards:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Stock Options Granted |  |  |  |  |
Expected term (in years) | 5.88 |  | 6.00 |  | 5.99
Expected volatility | 107 | % |  | 84 | % |  | 83 | %
Risk-free interest | 4.37 | % |  | 2.93 | % |  | 0.88 | %
Dividend yield | 0 | % |  | 0 | % |  | 0 | %
2016 ESPP |  |  |  |  |
Expected term (in years) | 0.49 |  | 0.49 |  | 0.50
Expected volatility | 181 | % |  | 84 | % |  | 86 | %
Risk-free interest | 4.99 | % |  | 1.95 | % |  | 0.08 | %
Dividend yield | 0 | % |  | 0 | % |  | 0 | %

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (e) -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 718 -SubTopic 10 -Subparagraph (f)(2) -Name Accounting Standards Codification -Paragraph 2 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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