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STOCKHOLDERS' EQUITY (Tables) | 12 Months Ended
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Dec. 31, 2023
Stockholders' Equity Note [Abstract] |
Schedule of aggregate stock option and award activity | Aggregate stock option and award activity for the 2011 Plan, 2018 Plan, 2006 Directors Plan, Inducement Plan and Directors Market Plan is as follows:
|  |  |  |  | Outstanding Stock Options |
|  |  |  |  |  |  |  |  |  |  | Weighted Average |  |  |  |
|  | Shares |  |  |  |  |  | Weighted Average |  |  | Remaining |  |  | Aggregate |
|  | Available |  |  | Number of |  |  | Exercise Price |  |  | Contractual Life |  |  | Intrinsic |
|  | For Grant |  |  | Shares |  |  | Per Share |  |  | (In years) |  |  | Value |
Balance at December 31, 2022 |  |  | 18,370,729 |  |  |  | 65,902,400 |  |  | $ | 1.87 |  |  |  |  |  |  |
Additional shares authorized |  |  | 56,368,058 |  |  |  | — |  |  | $ | — |  |  |  |  |  |  |
Stock options granted |  |  | (20,855,230 | ) |  |  | 20,855,230 |  |  | $ | 2.72 |  |  |  |  |  |  |
Awards granted |  |  | (36,864 | ) |  |  | — |  |  | $ | — |  |  |  |  |  |  |
Stock options exercised |  |  | — |  |  |  | (8,869,302 | ) |  | $ | 1.39 |  |  |  |  |  |  |
Stock options cancelled/forfeited/expired |  |  | 4,903,977 |  |  |  | (4,903,977 | ) |  | $ | 2.15 |  |  |  |  |  |  |
Balance at December 31, 2023 |  |  | 58,750,670 |  |  |  | 72,984,351 |  | (1) | $ | 2.16 |  |  |  | 6.70 |  |  | $ | 25,391,643 |
Stock options exercisable at December 31, 2023 |  |  |  |  |  | 39,995,642 |  |  | $ | 2.16 |  |  |  | 5.28 |  |  | $ | 15,557,976 |
Stock options fully vested and expected to vest at December 31, 2023 |  |  |  |  |  | 71,983,176 |  |  | $ | 2.15 |  |  |  | 6.67 |  |  | $ | 25,169,074 |
(1) Include s 7,936,030 perfo rmance-based stock options granted that have not achieved the specified performance milestones.
Summary of allocation of stock-based compensation expense related to share-based payment awards | The following table summarizes the stock‑based compensation expense related to service-based stock options and employee stock purchases for the years ended December 31, 2023, 2022 and 2021, which was allocated as follows:
|  | Year Ended December 31, |
(In thousands) |  | 2023 |  |  | 2022 |  |  | 2021 |
Research and development |  | $ | 7,426 |  |  | $ | 3,720 |  |  | $ | 3,597 |
General and administrative |  |  | 11,099 |  |  |  | 4,281 |  |  |  | 4,483 |
Stock-based compensation expense included in operating expenses |  | $ | 18,525 |  |  | $ | 8,001 |  |  | $ | 8,080 |
Schedule of assumptions used to estimate the fair value of stock options granted | The fair value of stock options granted in 2023, 2022, and 2021 has been estimated at the date of grant using the Black-Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2023 |  | 2022 |  | 2021
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.815 to 0.827 |  | 0.772 to 0.817 |  | 0.775 to 0.783
Risk-free interest rate range |  | 3.42% to 4.94% |  | 1.69% to 4.57% |  | 0.51% to 1.30%
Expected term range |  | 6.0 yrs |  | 5.5 yrs |  | 5.5 yrs
Schedule of assumptions used to estimate the fair value of employee stock purchases under the purchase plan | The fair value of employee stock purchases in 2023, 2022, and 2021 has been estimated using the Black-Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2023 |  | 2022 |  | 2021
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.791 to 0.832 |  | 0.614 to 0.865 |  | 0.507 to 0.707
Risk-free interest rate range |  | 4.73% to 5.4% |  | 0.40% to 2.79% |  | 0.09% to 0.16%
Expected term range |  | 6 - 12 mos |  | 6 - 12 mos |  | 6 - 12 mos
Schedule of common stock reserved for future issuance | Common stock reserved for future issuance as of December 31, 2023 is as follows:
Outstanding stock options |  |  | 72,984,351 |
Stock options and awards available for grant |  |  | 58,750,670 |
Employee stock purchase plan |  |  | 745,838 |
Warrants outstanding |  |  | 53,904,980 |
Total |  |  | 186,385,839 |

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