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Equity Incentive Plans (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Schedule of Stock Option Activity | A summary of our stock option activity and related information during the year ended December 31, 2023 is as follows (in thousands, except per share dollar amounts and years):
|  |  | Options Issued and Outstanding |  | Weighted Average Remaining Contractual Term (in Years) |  | Aggregate Intrinsic Value
| Number of Shares |  | Weighted-Average Exercise Price per Share |  |
Balance at December 31, 2022 |  |  | 13,963 |  |  | $ | 4.83 |  |  |  |  |
Options granted |  |  | 8,914 |  |  | $ | 3.14 |  |  |  |  |
Options exercised |  |  | (226) |  |  | $ | 1.61 |  |  |  |  |
Options canceled |  |  | (483) |  |  | $ | 3.83 |  |  |  |  |
Balance at December 31, 2023 |  |  | 22,168 |  |  | $ | 4.20 |  |  | 7.3 |  | $ | 58,606 |
Vested and expected to vest at December 31, 2023 |  |  | 22,168 |  |  | $ | 4.20 |  |  | 7.3 |  | $ | 58,606 |
Exercisable at December 31, 2023 |  |  | 12,199 |  |  | $ | 5.30 |  |  | 6.1 |  | $ | 25,116 |
Schedule of Valuation Assumptions for Stock Options | The estimated grant date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following weighted-average assumptions:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Expected term (years) | 5.1 |  | 4.9 |  | 5.0
Expected volatility | 97.6 | % |  | 92.1 | % |  | 77.0 | %
Risk-free interest rate | 3.8 | % |  | 2.2 | % |  | 4.7 | %
Dividend yield | — | % |  | — | % |  | — | %
Schedule of Restricted Stock Units Activity | A summary of our RSUs activity and related information for the year ended December 31, 2023 is as follows (in thousands, except per share dollar amounts):
| Number of RSUs |  | Weighted-Average Grant Date Fair Value Per Share |  |  |  |
Non-vested restricted stock units at December 31, 2022 | 1,406 |  |  | $ | 2.17 |  |  |  |  |
Granted | 3,269 |  |  | $ | 3.39 |  |  |  |  |
Vested | (942) |  |  | $ | 2.76 |  |  |  |  |
Forfeited | (87) |  |  | $ | 3.05 |  |  |  |  |
Non-vested restricted stock units at December 31, 2023 | 3,646 |  |  | $ | 3.09 |  |  |  |  |
Schedule of Valuation Assumptions for Employee Stock Purchase Plan | The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of ESPP purchase rights granted to our employees:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Expected term (years) | 0.5 |  | 0.5 |  | 0.5
Expected volatility | 86.0 | % |  | 97.2 | % |  | 123.0 | %
Risk-free interest rate | 5.3 | % |  | 1.9 | % |  | 0.7 | %
Dividend yield | — | % |  | — | % |  | — | %
Schedule of Stock-Based Compensation Recognized | Stock-based compensation expense recognized for stock options, RSUs, and our ESPP are recorded as operating expenses in our statements of operations and comprehensive loss, as follows (in thousands):
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Selling, general and administrative | $ | 9,952 |  |  | $ | 7,525 |  |  | $ | 7,923 |
Research and development | 3,578 |  |  | 3,225 |  |  | 4,116 |
Total | $ | 13,530 |  |  | $ | 10,750 |  |  | $ | 12,039 |
Schedule of Total Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures | A summary of our total unrecognized stock-based compensation expense, net of estimated forfeitures, as of December 31, 2023 is as follows (dollars in thousands):
| December 31, 2023
| Unrecognized Compensation Expense |  | Average Remaining Vesting Period (Years)
Stock option grants | $ | 19,960 |  |  | 2.75
RSU grants | $ | 10,579 |  |  | 3.03
ESPP | $ | 154 |  |  | 0.2

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