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STOCKHOLDERS' EQUITY (Tables) | 12 Months Ended
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Dec. 31, 2019
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, 2018 |  |  | 11,964,399 |  |  |  | 27,285,952 |  |  | $ | 2.72 |  |  |  |  |  |  |  |  |
Additional shares authorized |  |  | 5,000,000 |  |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Options granted |  |  | (11,735,600 | ) |  |  | 11,735,600 |  | (1) | $ | 1.32 |  |  |  |  |  |  |  |  |
Awards granted |  |  | (29,150 | ) |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Options exercised |  |  | — |  |  |  | (137,333 | ) |  | $ | 1.19 |  |  |  |  |  |  |  |  |
Options cancelled/forfeited/expired |  |  | 1,207,706 |  |  |  | (1,270,206 | ) |  | $ | 3.50 |  |  |  |  |  |  |  |  |
Balance at December 31, 2019 |  |  | 6,407,355 |  | (2) |  | 37,614,013 |  | (3) | $ | 2.26 |  |  |  | 6.83 |  |  | $ | 1,800 |
Options exercisable at December 31, 2019 |  |  |  |  |  |  | 19,915,713 |  |  | $ | 2.86 |  |  |  | 4.95 |  |  | $ | 359 |
Options fully vested and expected to vest at December 31, 2019 |  |  |  |  |  |  | 36,293,713 |  |  | $ | 2.29 |  |  |  | 6.75 |  |  | $ | 1,698 |
(1) | Includes 1,000,000 performance-based stock options granted in 2019 that have not achieved certain strategic milestones.
(2) | In February 2020, our Compensation Committee approved an amendment to increase the reserve for the 2018 Inducement Plan from 8,000,000 to 9,300,000 shares of common stock.
(3) | Includes 1,000,000 and 4,500,000 performance-based stock options granted in 2019 and 2018, respectively, that have not achieved certain strategic milestones.
Schedule of information about stock options outstanding by exercise price range | Information about stock options outstanding as of December 31, 2019 is as follows:
|  | Options Outstanding |
|  |  |  |  |  |  |  |  |  | Weighted Average |
|  |  |  |  |  | Weighted Average |  |  | Remaining |
|  | Number of |  |  | Exercise Price |  |  | Contractual Life |
Exercise Price Range |  | Shares |  |  | Per Share |  |  | (In years) |
$1.03 - $1.48 |  |  | 9,578,200 |  |  | $ | 1.19 |  |  |  | 8.26 |
$1.50 - $1.74 |  |  | 10,928,400 |  |  | $ | 1.67 |  |  |  | 7.17 |
$1.77 - $2.54 |  |  | 9,732,417 |  |  | $ | 2.27 |  |  |  | 6.86 |
$2.63 - $5.29 |  |  | 7,374,996 |  |  | $ | 4.54 |  |  |  | 4.43 |
$1.03 - $5.29 |  |  | 37,614,013 |  | (1) | $ | 2.26 |  |  |  | 6.83 |
| (1) | Includes 1,000,000 and 4,500,000 performance-based stock options granted in 2019 and 2018, respectively, that have not achieved certain strategic milestones.
Summary of allocation of stock-based compensation expense related to share-based payment awards |
|  | Year Ended December 31, |
(In thousands) |  | 2019 |  |  | 2018 |  |  | 2017 |
Research and development |  | $ | 1,640 |  |  | $ | 949 |  |  | $ | 988 |
General and administrative |  |  | 4,439 |  |  |  | 5,419 |  |  |  | 7,156 |
Stock-based compensation expense included in operating expenses |  | $ | 6,079 |  |  | $ | 6,368 |  |  | $ | 8,144 |
Schedule of assumptions used to estimate the fair value of stock options granted | The fair value of stock options granted in 2019, 2018 and 2017 has been estimated at the date of grant using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2019 |  | 2018 |  | 2017
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.792 to 0.980 |  | 0.821 to 0.990 |  | 0.884 to 0.892
Risk-free interest rate range |  | 1.50% to 2.56% |  | 2.55% to 3.11% |  | 1.98% to 1.99%
Expected term range |  | 5.25 - 6.44 yrs |  | 5.25 - 6.62 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 2019, 2018 and 2017 has been estimated using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2019 |  | 2018 |  | 2017
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.646 to 1.653 |  | 0.437 to 0.475 |  | 0.577 to 0.641
Risk-free interest rate range |  | 1.94% to 2.63% |  | 1.53% to 1.76% |  | 0.45% to 0.89%
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, 2019 is as follows:
Outstanding stock options |  |  | 37,614,013 |
Options and awards available for grant |  |  | 6,407,355 |
Employee stock purchase plan |  |  | 836,359 |
Warrant outstanding |  |  | 537,893 |
Total |  |  | 45,395,620 |

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