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STOCKHOLDERS' EQUITY (Tables) | 12 Months Ended
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Dec. 31, 2020
Stockholders Equity Note [Abstract] |
Schedule of aggregate option and award activity | Aggregate option and award activity for the 2002 Plan, 2011 Plan, 2018 Plan, 2006 Directors Plan, Inducement Plan and Directors Market Plan is as follows:
|  |  |  |  |  | Outstanding Options |
|  |  |  |  |  |  |  |  |  |  |  |  |  | Weighted Average |  |  | Aggregate |
|  | Shares |  |  |  |  |  |  | Weighted Average |  |  | Remaining |  |  | Intrinsic |
|  | Available |  |  | Number of |  |  | Exercise Price |  |  | Contractual Life |  |  | Value |
|  | For Grant |  |  | Shares |  |  | Per Share |  |  | (In years) |  |  | (In thousands) |
Balance at December 31, 2019 |  |  | 6,407,355 |  |  |  | 37,614,013 |  |  | $ | 2.26 |  |  |  |  |  |  |  |  |
Additional shares authorized |  |  | 7,000,000 |  |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Options granted |  |  | (6,531,931 | ) |  |  | 6,531,931 |  |  | $ | 1.37 |  |  |  |  |  |  |  |  |
Awards granted |  |  | (17,986 | ) |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Options exercised |  |  | — |  |  |  | (21,944 | ) |  | $ | 1.16 |  |  |  |  |  |  |  |  |
Options cancelled/forfeited/expired |  |  | 400,309 |  |  |  | (460,309 | ) |  | $ | 3.48 |  |  |  |  |  |  |  |  |
Balance at December 31, 2020 |  |  | 7,257,747 |  | (1) |  | 43,663,691 |  | (2) | $ | 2.12 |  |  |  | 6.32 |  |  | $ | 5,624 |
Options exercisable at December 31, 2020 |  |  |  |  |  |  | 25,721,508 |  |  | $ | 2.54 |  |  |  | 4.90 |  |  | $ | 2,247 |
Options fully vested and expected to vest at December 31, 2020 |  |  |  |  |  |  | 42,621,956 |  |  | $ | 2.13 |  |  |  | 6.27 |  |  | $ | 5,437 |
(1) | In February 2021, our Compensation Committee approved an amendment to increase the reserve for the Inducement Plan from 9,300,000 to 10,100,000 shares of common stock.
(2) | Includes 5,500,000 performance-based stock options granted previously that have not achieved certain strategic 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, restricted stock awards and employee stock purchases for the years ended December 31, 2020, 2019 and 2018 which was allocated as follows:
|  | Year Ended December 31, |
(In thousands) |  | 2020 |  |  | 2019 |  |  | 2018 |
Research and development |  | $ | 2,337 |  |  | $ | 1,640 |  |  | $ | 949 |
General and administrative |  |  | 4,558 |  |  |  | 4,439 |  |  |  | 5,419 |
Stock-based compensation expense included in operating expenses |  | $ | 6,895 |  |  | $ | 6,079 |  |  | $ | 6,368 |
Schedule of assumptions used to estimate the fair value of stock options granted | The fair value of stock options granted in 2020, 2019 and 2018 has been estimated at the date of grant using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2020 |  | 2019 |  | 2018
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.781 to 0.793 |  | 0.792 to 0.980 |  | 0.821 to 0.990
Risk-free interest rate range |  | 0.31% to 1.62% |  | 1.50% to 2.56% |  | 2.55% to 3.11%
Expected term range |  | 5.25 yrs |  | 5.25 - 6.44 yrs |  | 5.25 - 6.62 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 2020, 2019 and 2018 has been estimated using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2020 |  | 2019 |  | 2018
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.478 to 0.818 |  | 0.646 to 1.653 |  | 0.437 to 0.475
Risk-free interest rate range |  | 0.16% to 1.57% |  | 1.94% to 2.63% |  | 1.53% to 1.76%
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, 2020 is as follows:
Outstanding stock options |  |  | 43,663,691 |
Options and awards available for grant |  |  | 7,257,747 |
Employee stock purchase plan |  |  | 682,508 |
Warrants outstanding |  |  | 66,552,939 |
Total |  |  | 118,156,885 |

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