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Stock-Based Compensation | 3 Months Ended
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Mar. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
STOCK-BASED COMPENSATION | STOCK-BASED COMPENSATION We recorded and allocated employee stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (“ESPP”) as follows (in thousands):
| Three Months Ended March 31,
| 2018 |  | 2017
Research and development | $ | 3,033 |  |  | $ | 1,478 |
Selling, general and administrative | 6,272 |  |  | 3,235 |
Total stock-based compensation | $ | 9,305 |  |  | $ | 4,713 |
We have several equity incentive plans under which we have granted stock options and restricted stock units (“RSUs”) to employees, directors and consultants. At March 31, 2018, 19,972,317 shares were available for grant under our equity incentive plans. We use the Black-Scholes Merton option pricing model to value our stock options and ESPP purchases. The weighted average grant-date fair value per share of our stock options and ESPP purchases was as follows:
| Three Months Ended March 31,
| 2018 |  | 2017
Stock options | $ | 11.52 |  |  | $ | 9.92 |
ESPP | $ | 7.39 |  |  | $ | 3.71 |
The grant-date fair value of employee stock option grants and ESPP purchases was estimated using the following assumptions:
| Three Months Ended March 31,
| 2018 |  | 2017
Stock options: |  |  |
Risk-free interest rate | 2.40 | % |  | 1.62 | %
Dividend yield | — | % |  | — | %
Volatility | 54 | % |  | 64 | %
Expected life | 4.0 years |  |  | 4.0 years |
ESPP: |  |  |
Risk-free interest rate | 1.53 | % |  | 0.62 | %
Dividend yield | — | % |  | — | %
Volatility | 53 | % |  | 68 | %
Expected life | 6 months |  |  | 6 months |
We considered our implied volatility and our historical volatility in developing our estimates of expected volatility. The assumptions for the expected life of stock options were based on historical exercise patterns and post-vesting termination behavior. The fair value of RSUs was based on the closing price of the underlying common stock on the date of grant. Stock option activity for the three months ended March 31, 2018 was as follows (dollars in thousands, except per share amounts):
| Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
Options outstanding at December 31, 2017 | 22,208,446 |  |  | $ | 6.83 |  |  |  |  |
Granted | 293,580 |  |  | $ | 25.72 |  |  |  |  |
Exercised | (288,196 | ) |  | $ | 6.69 |  |  |  |  |
Forfeited | (18,484 | ) |  | $ | 12.04 |  |  |  |  |
Options outstanding at March 31, 2018 | 22,195,346 |  |  | $ | 7.08 |  |  | 3.86 years |  | $ | 339,631 |
Exercisable at March 31, 2018 | 16,497,014 |  |  | $ | 4.66 |  |  | 3.30 years |  | $ | 288,535 |
As of March 31, 2018, $38.8 million of unrecognized compensation expense related to unvested stock options will be recognized over a weighted-average period of 2.47 years. RSU activity for the three months ended March 31, 2018 was as follows (dollars in thousands, except per share amounts):
| Shares |  | Weighted Average Grant Date Fair Value Per Share |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
RSUs outstanding at December 31, 2017 | 3,762,990 |  |  | $ | 17.76 |  |  |  |  |
Awarded | 146,790 |  |  | $ | 25.72 |  |  |  |  |
Vested and released | (197,884 | ) |  | $ | 6.30 |  |  |  |  |
Forfeited | (24,494 | ) |  | $ | 17.89 |  |  |  |  |
RSUs outstanding at March 31, 2018 | 3,687,402 |  |  | $ | 18.69 |  |  | 1.83 years |  | $ | 81,676 |
As of March 31, 2018, $60.0 million of unrecognized compensation expense related to unvested RSUs will be recognized over a weighted-average period of 3.05 years.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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