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Stock-based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2018
Stock-based Compensation |
Schedule of stock-based compensation expense | Total compensation cost recognized for all stock‑based compensation awards in the Company’s condensed consolidated statements of operations was as follows (in thousands):
|  | Three Months Ended |  | Nine Months Ended
|  | September 30, |  | September 30,
|  | 2018 |  | 2017 |  | 2018 |  | 2017
Research and development |  | $ | 3,506 |  | $ | 2,431 |  | $ | 11,412 |  | $ | 9,130
General and administrative |  |  | 3,193 |  |  | 2,037 |  |  | 8,839 |  |  | 6,157
Total stock-compensation expense |  | $ | 6,699 |  | $ | 4,468 |  | $ | 20,251 |  | $ | 15,287
Schedule of changes in unvested restricted stock |
|  |  |  | Weighted
|  |  |  | Average
|  |  |  | Grant Date
|  |  |  | Fair Value
|  | Shares |  | Per Share
Unvested Restricted Common Stock as of December 31, 2017 |  | 513,225 |  | $ | 18.70
Issued |  | — |  |  | —
Vested |  | (225,225) |  | $ | 6.75
Forfeited |  | — |  |  | —
Unvested Restricted Common Stock as of September 30, 2018 |  | 288,000 |  | $ | 28.05
Schedule of stock option activity |
|  |  |  | Weighted Average |  | Remaining |  | Aggregate Intrinsic
|  | Shares |  | Exercise Price |  | Contractual Life (years) |  | Value (in thousands)
Outstanding at December 31, 2017 |  | 4,372,136 |  | $ | 17.28 |  | 8.5 |  | $ | 60,591
Granted |  | 1,643,185 |  | $ | 36.29 |  | — |  |  | —
Exercised |  | (634,331) |  | $ | 13.73 |  | — |  |  | —
Cancelled |  | (558,504) |  | $ | 24.88 |  | — |  |  | —
Outstanding at September 30, 2018 |  | 4,822,486 |  | $ | 23.38 |  | 8.2 |  | $ | 48,750
Exercisable at September 30, 2018 |  | 1,881,620 |  | $ | 17.65 |  | 7.5 |  | $ | 28,201
Schedule of assumptions used to value stock options |
|  | Three Months Ended |  | Nine Months Ended |
|  | September 30, |  | September 30, |
|  | 2018 |  | 2017 |  | 2018 |  |  | 2017 |
Risk free interest rate |  | 2.9 | % |  | 1.9 | % |  | 2.6 | % |  | 2.1 | %
Expected dividend yield |  | — |  |  | — |  |  | — |  |  | — |
Expected term (in years) |  | 6.25 |  |  | 6.25 |  |  | 6.25 |  |  | 6.25 |
Expected volatility |  | 77.3 | % |  | 78.0 | % |  | 79.8 | % |  | 77.9 | %

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