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Stockholders' Equity (Tables) | 12 Months Ended
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Dec. 31, 2017
Stock-Based Compensation Expense | The following table presents stock-based compensation expense in the Company’s consolidated statements of operations (in thousands):
|  | Years ended December 31, |
|  | 2017 |  |  | 2016 |  |  | 2015 |
Cost of product revenue |  | $ | 704 |  |  | $ | 341 |  |  | $ | 213 |
Research and development |  |  | 481 |  |  |  | 537 |  |  |  | 336 |
Selling, general and administrative |  |  | 5,562 |  |  |  | 3,717 |  |  |  | 3,049 |
Total |  | $ | 6,747 |  |  | $ | 4,595 |  |  | $ | 3,598 |
Estimated Weighted Average Assumptions | The fair value of share-based awards granted during the years ended December 31, 2017, 2016 and 2015 were calculated using the following estimated assumptions:
|  | 2017 |  | 2016 |  | 2015
Expected term (years) |  | 6.1 |  | 6.7 – 7.1 |  | 6.6 – 7.2
Volatility |  | 51.48% |  | 50.85 – 51.01% |  | 50.09 – 51.89%
Risk-free interest rate |  | 1.88 – 1.99% |  | 1.51 – 2.37% |  | 1.67 – 2.03%
Expected dividend yield |  | — |  | — |  | —
Summary of Option Activity | Information regarding option activity for the year ended December 31, 2017 under the Plans is summarized below:
|  | Options Outstanding |  |  | Weighted- Average Exercise Price Per Share |  |  | Weighted- Average Remaining Contractual Term (in years) |  |  | (in thousands) Aggregate Intrinsic Value |
Options outstanding at December 31, 2016 |  |  | 882,748 |  |  | $ | 16.88 |  |  |  |  |  |  |  |  |
Granted |  |  | 101,844 |  |  |  | 33.38 |  |  |  |  |  |  |  |  |
Exercised |  |  | (215,495 | ) |  |  | 10.92 |  |  |  |  |  |  |  |  |
Forfeited/cancelled |  |  | (34,157 | ) |  |  | 20.23 |  |  |  |  |  |  |  |  |
Options outstanding at December 31, 2017 |  |  | 734,940 |  |  | $ | 20.80 |  |  |  | 6.46 |  |  | $ | 11,558 |
Options exercisable at December 31, 2017 |  |  | 420,688 |  |  | $ | 15.86 |  |  |  | 5.28 |  |  | $ | 8,720 |
Vested and expected to vest at December 31, 2017(1) |  |  | 727,278 |  |  | $ | 20.71 |  |  |  | 6.44 |  |  | $ | 11,472 |
(1) | Represents the number of vested options as of December 31, 2017 plus the number of unvested options expected to vest as of December 31, 2017 based on the unvested outstanding options at December 31, 2017 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.
Summary of Restricted Stock Unit Activity | Information regarding restricted stock unit activity for the year ended December 31, 2017 under the Plans is summarized below:
|  | Options Outstanding |  |  | Weighted- Average Remaining Contractual Term (in years) |  |  | (in thousands) Aggregate Intrinsic Value |
Restricted stock units outstanding at December 31, 2016 |  |  | 353,838 |  |  |  |  |  |  |  |  |
Granted |  |  | 293,004 |  |  |  |  |  |  |  |  |
Vested |  |  | (114,690 | ) |  |  |  |  |  |  |  |
Forfeited/cancelled |  |  | (26,917 | ) |  |  |  |  |  |  |  |
Restricted stock units outstanding at December 31, 2017 |  |  | 505,235 |  |  |  | 2.65 |  |  | $ | 18,330 |
Vested and expected to vest at December 31, 2017(1) |  |  | 466,201 |  |  |  | 2.40 |  |  | $ | 16,914 |
(1) | Represents the number of vested restricted stock units as of December 31, 2017 plus the number of unvested restricted stock units expected to vest as of December 31, 2017 based on the unvested outstanding restricted stock units at December 31, 2017 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.

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