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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Stock-Based Compensation |
Summary of stock option activity |
|  | Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term (In Years) |
Options Outstanding at December 31, 2016 |  |  | 10,218,710 |  | $ | 11.14 |  |  | 6.5 |
Granted |  |  | 2,427,200 |  | $ | 2.37 |  |  |  |
Exercised |  |  | — |  | $ | — |  |  |  |
Canceled |  |  | (1,789,698 | ) | $ | 9.76 |  |  |  |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Options Outstanding at December 31, 2017 |  |  | 10,856,212 |  | $ | 9.40 |  |  | 6.1 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Options Vested and Expected to Vest at December 31, 2017 |  |  | 10,780,023 |  | $ | 9.43 |  |  | 6.1 |
Options Exercisable at December 31, 2017 |  |  | 6,846,254 |  | $ | 11.18 |  |  | 4.5 |
Shares Available for Grant Under the 2008 Plan |  |  | 8,324,310 |  |  |  |  |  |  |
Summary of restricted stock activity |
|  | Shares |  | Weighted Average Grant Date Fair Value (per share) |
Outstanding and unvested at December 31, 2016 |  |  | 50,000 |  | $ | 4.72 |
Granted |  |  | 70,000 |  | $ | 2.32 |
Vested |  |  | (16,665 | ) | $ | 4.72 |
Canceled |  |  | (6,667 | ) | $ | 4.72 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Outstanding and unvested at December 31, 2017 |  |  | 96,668 |  | $ | 2.98 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Schedule of stock-based compensation expense |
|  | 2017 |  | 2016 |  | 2015 |
|  | (In thousands) |
Research and development |  | $ | 6,693 |  | $ | 7,821 |  | $ | 6,186 |
General and administrative |  |  | 5,619 |  |  | 7,496 |  |  | 6,588 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total stock-based compensation expense |  | $ | 12,312 |  | $ | 15,317 |  | $ | 12,774 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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Schedule of assumptions used for the fair value of employee stock options granted |
|  | 2017 |  | 2016 |  | 2015
Expected stock price volatility |  | 75 - 77% |  | 70 - 77% |  | 67 - 69%
Expected option term |  | 6.0 Years |  | 6.0 Years |  | 6.0 Years
Risk-free interest rate |  | 2.0 - 2.3% |  | 1.4 - 2.3% |  | 1.8 - 2.2%
Expected dividend yield |  | None |  | None |  | None

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