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Stock-Based Compensation | 3 Months Ended
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Mar. 31, 2014
Stock-Based Compensation |
Stock-Based Compensation | (10) Stock-Based Compensation A summary of stock option activity for the three months ended March 31, 2014 is as follows:
|  | Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term (In Years) |
Options Outstanding at December 31, 2013 |  | 5,770,544 |  | $ | 8.17 |  | 7.0 |
Granted |  | 12,750 |  | $ | 26.37 |  |  |
Exercised |  | (107,814 | ) | $ | 6.10 |  |  |
Canceled |  | (7,527 | ) | $ | 10.46 |  |  |
Options Outstanding at March 31, 2014 |  | 5,667,953 |  | $ | 8.25 |  | 6.8 |
Options Vested and Expected to Vest at March 31, 2014 |  | 5,627,995 |  | $ | 8.21 |  | 6.8 |
Options Exercisable at March 31, 2014 |  | 3,371,583 |  | $ | 6.22 |  | 5.5 |
Shares Available for Grant under the 2008 Plan |  | 2,097,719 |  |  |  |  |
The weighted average grant-date fair value of stock options granted during the three months ended March 31, 2014 was $16.91. Stock-based compensation expense for the three months ended March 31, 2014 and 2013 was recorded as follows:
|  | Three months ended March 31, |
|  | 2014 |  | 2013 |
|  | (In thousands) |
Research and development |  | $ | 591 |  | $ | 441 |
General and administrative |  | 659 |  | 267 |
Total stock-based compensation expense |  | $ | 1,250 |  | $ | 708 |
The fair values of employee stock options granted during the three months ended March 31, 2014 and 2013 were valued using the Black-Scholes option-pricing model with the following assumptions:
|  | Three months ended March 31, |
|  | 2014 |  | 2013 |
Expected stock price volatility |  | 71 | % | 72 | %
Expected option term |  | 6.0 Years |  | 6.0 Years |
Risk-free interest rate |  | 2.2 | % | 1.4 | %
Expected dividend yield |  | None |  | None |

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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 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5444-113901 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 + Details
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