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STOCKHOLDERS' EQUITY (Tables) | 12 Months Ended
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Dec. 31, 2017
Stockholders Equity Note [Abstract] |
Schedule of aggregate option and award activity | Aggregate option and award activity for the 2002 Plan, 2011 Plan and 2006 Directors 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, 2016 |  |  | 9,570,535 |  |  |  | 19,125,287 |  |  | $ | 3.15 |  |  |  |  |  |  |
Options granted |  |  | (3,484,000 | ) |  |  | 3,484,000 |  |  | $ | 2.19 |  |  |  |  |  |  |
Awards granted |  |  | (72,066 | ) |  |  | — |  |  | $ | — |  |  |  |  |  |  |
Options exercised |  |  | — |  |  |  | (12,500 | ) |  | $ | 1.41 |  |  |  |  |  |  |
Options cancelled/forfeited |  |  | 188,258 |  |  |  | (188,258 | ) |  | $ | 7.99 |  |  |  |  |  |  |
Balance at December 31, 2017 |  |  | 6,202,727 |  |  |  | 22,408,529 |  |  | $ | 2.96 |  |  | 6.24 |  | $ | 2,034 |
Options exercisable at December 31, 2017 |  |  |  |  |  |  | 17,249,032 |  |  | $ | 3.03 |  |  | 5.57 |  | $ | 2,034 |
Options fully vested and expected to vest at December 31, 2017 |  |  |  |  |  |  | 22,171,142 |  |  | $ | 2.96 |  |  | 6.21 |  | $ | 2,034 |
Schedule of information about stock options outstanding by exercise price range | 8. STOCKHOLDERS’ EQUITY (Continued) Information about stock options outstanding as of December 31, 2017 is as follows:
|  | Options Outstanding |
|  |  |  |  |  |  |  |  |  | Weighted Average |
|  |  |  |  |  | Weighted Average |  |  | Remaining |
|  | Number of |  |  | Exercise Price |  |  | Contractual Life |
Exercise Price Range |  | Shares |  |  | Per Share |  |  | (In years) |
$1.10 - $1.51 |  |  | 5,655,662 |  |  | $ | 1.45 |  |  |  | 4.89 |
$1.55 - $2.16 |  |  | 5,631,031 |  |  | $ | 2.08 |  |  |  | 7.04 |
$2.22 - $4.34 |  |  | 6,000,783 |  |  | $ | 3.37 |  |  |  | 7.60 |
$4.42 - $7.31 |  |  | 5,121,053 |  |  | $ | 5.10 |  |  |  | 5.27 |
$1.10 - $7.31 |  |  | 22,408,529 |  |  | $ | 2.96 |  |  |  | 6.24 |
Schedule of aggregate restricted stock activity | Aggregate restricted stock activity for the 2011 Plan is as follows:
|  |  |  |  |  | Weighted Average |  |  | Weighted Average |
|  |  |  |  |  | Grant Date |  |  | Remaining |
|  | Number of |  |  | Fair Value |  |  | Contractual Term |
|  | Shares |  |  | Per Share |  |  | (In years) |
Non-vested restricted stock at December 31, 2016 |  |  | — |  |  | $ | — |  |  |  | — |
Granted |  |  | 72,066 |  |  | $ | 2.20 |  |  |  |  |
Vested |  |  | (72,066 | ) |  | $ | 2.20 |  |  |  |  |
Non-vested restricted stock at December 31, 2017 |  |  | — |  |  | $ | — |  |  |  | — |
Summary of allocation of stock-based compensation expense related to share-based payment awards |
|  | Year Ended December 31, |
(In thousands) |  | 2017 |  |  | 2016 |  |  | 2015 |
Research and development |  | $ | 988 |  |  | $ | 1,275 |  |  | $ | 2,139 |
Restructuring charges |  |  | — |  |  |  | — |  |  |  | 307 |
General and administrative |  |  | 7,156 |  |  |  | 6,970 |  |  |  | 5,951 |
Stock-based compensation expense included in operating expenses |  | $ | 8,144 |  |  | $ | 8,245 |  |  | $ | 8,397 |
Schedule of assumptions used to estimate the fair value of stock options granted | The fair value of stock options granted in 2017, 2016 and 2015 has been estimated at the date of grant using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2017 |  | 2016 |  | 2015
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.884 to 0.892 |  | 0.888 to 0.890 |  | 0.874 to 0.884
Risk-free interest rate range |  | 1.98% to 1.99% |  | 1.21% to 1.38% |  | 1.68% to 1.71%
Expected term |  | 5.5 yrs |  | 5.5 yrs |  | 5.5 yrs
Schedule of assumptions used to estimate the fair value of employee stock purchases under the purchase plan | 8. STOCKHOLDERS’ EQUITY (Continued) The fair value of employee stock purchases in 2017, 2016 and 2015 has been estimated using the Black Scholes option‑pricing model with the following assumptions:
|  | Year Ended December 31,
|  | 2017 |  | 2016 |  | 2015
Dividend yield |  | 0% |  | 0% |  | 0%
Expected volatility range |  | 0.577 to 0.641 |  | 0.641 to 0.684 |  | 0.654 to 1.392
Risk-free interest rate range |  | 0.45% to 0.89% |  | 0.28% to 0.45% |  | 0.11% to 0.28%
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, 2017 is as follows:
Outstanding stock options |  |  | 22,408,529 |
Options and awards available for grant |  |  | 6,202,727 |
Employee stock purchase plan |  |  | 895,308 |
Warrant outstanding |  |  | 537,893 |
Total |  |  | 30,044,457 |

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