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Stockholders' Equity (Tables) | 12 Months Ended
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Dec. 31, 2018
Stock-Based Compensation Expense | The following table presents stock-based compensation expense in the Company’s consolidated statements of operations and comprehensive income:
|  | For the Years Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
|  | (Amounts in thousands) |
Cost of product revenue |  | $ | 1,019 |  |  | $ | 704 |  |  | $ | 341 |
Research and development |  |  | 917 |  |  |  | 481 |  |  |  | 537 |
Selling, general and administrative |  |  | 8,256 |  |  |  | 5,562 |  |  |  | 3,717 |
Total stock-based compensation |  | $ | 10,192 |  |  | $ | 6,747 |  |  | $ | 4,595 |
Estimated Weighted Average Assumptions | The fair value of share-based awards granted during the years ended December 31, 2018, 2017 and 2016 were calculated using the following estimated assumptions:
|  | For the Years Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
Expected term (in years) |  |  | 5.5 – 7.5 |  |  |  | 6.1 |  |  |  | 6.7 – 7.1 |
Expected volatility (range) |  |  | 45.14 – 50.87% |  |  |  | 51.48% |  |  |  | 50.85 – 51.01% |
Risk-free interest rate |  |  | 2.63 – 2.96% |  |  |  | 1.88 – 1.99% |  |  |  | 1.51 – 2.37% |
Expected dividend yield |  |  | 0% |  |  |  | 0% |  |  |  | 0 % |
Summary of Option Activity | Information regarding option activity for the year ended December 31, 2018 under the Plans is summarized below:
|  | Shares |  |  | Weighted average exercise price |  |  | Weighted- Average Remaining Contractual Term (in Years) |  |  | Aggregate Intrinsic Value (in Thousands) |
Options outstanding at December 31, 2017 |  |  | 734,940 |  |  | $ | 20.80 |  |  |  |  |  |  |  |  |
Granted |  |  | 449,678 |  |  | $ | 35.48 |  |  |  |  |  |  |  |  |
Exercised |  |  | (176,804 | ) |  | $ | 19.31 |  |  |  |  |  |  |  |  |
Forfeited/expired/cancelled |  |  | (9,588 | ) |  | $ | 29.71 |  |  |  |  |  |  |  |  |
Options outstanding at December 31, 2018 |  |  | 998,226 |  |  | $ | 27.54 |  |  |  | 7.19 |  |  | $ | 25,273 |
Options exercisable at December 31, 2018 |  |  | 410,760 |  |  | $ | 18.53 |  |  |  | 4.96 |  |  | $ | 14,051 |
Vested and expected to vest at December 31, 2018 (1) |  |  | 953,454 |  |  |  |  |  |  |  | 7.10 |  |  | $ | 24,490 |
(1) | Represents the number of vested options as of December 31, 2018 plus the number of unvested options expected to vest as of December 31, 2018 based on the unvested outstanding options at December 31, 2018 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 RSU activity for the year ended December 31, 2018 under the Plans is summarized below:
|  | Shares |  |  | Weighted- Average Remaining Contractual Term (in Years) |  |  | Aggregate Intrinsic Value (in Thousands) |
Unvested at December 31, 2017 |  |  | 505,235 |  |  |  |  |  |  |  |  |
Awarded |  |  | 407,961 |  |  |  |  |  |  |  |  |
Vested |  |  | (153,383 | ) |  |  |  |  |  |  |  |
Forfeited/expired/cancelled |  |  | (54,400 | ) |  |  |  |  |  |  |  |
Unvested at December 31, 2018 |  |  | 705,413 |  |  |  | 3.98 |  |  | $ | 37,203 |
Vested and expected to vest at December 31, 2018 (1) |  |  | 652,543 |  |  |  | 3.26 |  |  | $ | 34,415 |
(1) | Represents the number of vested RSUs units as of December 31, 2018 plus the number of unvested RSUs expected to vest as of December 31, 2018 based on the unvested outstanding RSUs at December 31, 2018 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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