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Stock-Based Compensation | 12 Months Ended
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Dec. 31, 2015
Stock-Based Compensation | Note 13. Stock-Based Compensation Employee Stock Plans Employee Stock Purchase Plan The Company maintains an Employee Stock Purchase Plan (the “Purchase Plan”), which is intended to qualify as an employee stock purchase plan within the meaning of Section 423(b) of the Internal Revenue Code. Under the Purchase Plan, the Company’s Board of Directors may authorize participation by eligible employees, including officers, in periodic offerings. Under the Purchase Plan eligible employee participants may purchase shares of common stock of the Company at a purchase price equal to 85% of the lower of the fair market value per share on the start date of the offering period or the fair market value per share on the purchase date. The Purchase Plan consists of a fixed offering period of 12 months with two purchase periods within each offering period. The Purchase Plan was authorized to issue an aggregate of 1,320,500 shares. On June 10, 2015, the Company’s stockholders approved an amendment and restatement of the Purchase Plan that increased the aggregate number of shares of common stock authorized for issuance under the Purchase Plan by 1,500,000 shares. At December 31, 2015, the Company had 1,652,972 shares available for future issuance. 2008 Equity Incentive Plan The Company also maintains an equity compensation plan to provide long-term incentives for employees, contractors, and members of its Board of Directors. The Company currently grants equity awards from one plan, the 2008 Equity Incentive Plan (the “2008 Plan”). The 2008 Plan allows for the issuance of non-statutory and incentive stock options, restricted stock, restricted stock units, stock appreciation rights, other stock-related awards, and performance awards which may be settled in cash, stock, or other property. On June 6, 2012 and June 12, 2013, the stockholders approved amendments to the 2008 Plan (collectively the “Amended 2008 Plan”) such that the Amended 2008 Plan has reserved for issuance an amount not to exceed 19,540,940 shares. On June 10, 2015, the Company’s stockholders approved an amendment and restatement of the 2008 Plan that increased the aggregate number of shares of common stock authorized for issuance under the 2008 Plan by 5,000,000 shares. Awards under the Amended 2008 Plan generally have a maximum term of 10 years from the date of the award. The Amended 2008 Plan generally requires options to be granted at 100% of the fair market value of the Company’s common stock subject to the option on the date of grant and will generally vest over four years. Performance-based stock or cash awards granted under the Amended 2008 Plan are limited to either 500,000 shares of common stock or $1.0 million per recipient per calendar year. The attainment of any performance-based awards granted shall be conclusively determined by a committee designated by the Company’s Board of Directors. At December 31, 2015, no performance-based stock options were outstanding. At December 31, 2015, the Company had an aggregate of approximately 21.5 million shares of its common stock subject to outstanding options or remaining available for future issuance under the Amended 2008 Plan, of which approximately 14.1 million shares were subject to outstanding options, and approximately 7.4 million shares were available for future issuance under the Amended 2008 Plan. The Company’s policy is to issue new shares of common stock upon the exercise of options. Activity under the Company’s equity incentive plans related to stock options is set forth below (in thousands except weighted average exercise price):
|  | Number of Options Outstanding |  |  | Weighted Average Exercise Price per Share |
Balances at December 31, 2014 |  |  | 11,323 |  |  | $ | 4.13 |
Granted |  |  | 3,730 |  |  |  | 4.58 |
Forfeited |  |  | (225 | ) |  |  | 5.12 |
Expired |  |  | (223 | ) |  |  | 8.55 |
Exercised |  |  | (486 | ) |  |  | 2.74 |
Balances at December 31, 2015 |  |  | 14,119 |  |  |  | 4.21 |
Information regarding the Company’s stock options outstanding, stock options vested and expected to vest, and stock options exercisable at December 31, 2015, was as follows (in thousands except weighted average exercise price and contractual term):
|  | Number of Shares |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Term (Years) |  |  | Aggregate Intrinsic Value |
Balances at December 31, 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options outstanding |  |  | 14,119 |  |  | $ | 4.21 |  |  |  | 6.9 |  |  | $ | 30,746 |
Stock options vested and expected to vest |  |  | 13,606 |  |  |  | 4.19 |  |  |  | 6.8 |  |  |  | 29,973 |
Stock options exercisable |  |  | 8,693 |  |  |  | 3.81 |  |  |  | 5.7 |  |  |  | 22,754 |
The aggregate intrinsic value in the table above is calculated as the difference between the exercise price of the stock option and the Company’s closing stock price on the last trading day of each respective fiscal period. The total intrinsic value of options exercised for the years ended December 31, 2015, 2014 and 2013, was $1.2 million, $3.8 million and $0.6 million, respectively. Stock-based Compensation Expense Stock-based compensation expense recognized on the Company’s consolidated statements of operations for the years ended December 31, 2015, 2014 and 2013, was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2015 |  |  | 2014 |  |  | 2013 |
Stock-based compensation expense by caption: |  |  |  |  |  |  |  |  |  |  |  |
Research and development |  | $ | 1,260 |  |  | $ | 998 |  |  | $ | 482 |
Selling, general and administrative |  |  | 5,470 |  |  |  | 4,155 |  |  |  | 2,786 |
Total stock-based compensation expense |  | $ | 6,730 |  |  | $ | 5,153 |  |  | $ | 3,268 |
Stock-based compensation expense in the above table does not reflect any income taxes as the Company has experienced a history of net losses since its inception and has a full valuation allowance on its deferred tax assets. In addition, there was neither income tax benefits realized related to stock-based compensation expense nor any stock-based compensation costs capitalized as part of an asset during the years ended December 31, 2015, 2014 and 2013. The Company has also not recorded any stock-based compensation associated with performance-based stock options during the years ended December 31, 2015, 2014 and 2013. As of December 31, 2015, the Company expects to recognize the remaining unamortized stock-based compensation expense of $11.0 million related to non-vested stock options, net of estimated forfeitures, over an estimated remaining weighted average period of 2.6 years. Valuation Assumptions for Stock-based Compensation The Company currently uses the Black-Scholes option pricing model to determine the grant-date fair value of stock options and employee stock purchase plan shares. The Black-Scholes option-pricing model is affected by the Company’s stock price, as well as assumptions regarding a number of complex and subjective variables, which include the expected term of the grants, actual and projected employee stock option exercise behaviors, including forfeitures, the Company’s expected stock price volatility, the risk-free interest rate and expected dividends. The Company recognizes the grant-date fair value of the stock award as stock-based compensation expense on a straight-line basis over the requisite service period, which is the vesting period, and is adjusted for estimated forfeitures. The expected life of the stock options is based on observed historical exercise patterns. Groups of employees having similar historical exercise behavior are considered separately for valuation purposes. The Company estimates stock option forfeitures based on historical data for employee groups. The total number of stock options expected to vest is adjusted by actual and estimated forfeitures. The expected volatility is estimated by using historical volatility of the Company’s common stock. The risk-free interest rate is based on the implied yield on a U.S. Treasury zero-coupon issue with a remaining term commensurate with the expected term of the option. The Company does not anticipate paying any cash dividends in the foreseeable future and therefore uses an expected dividend yield of zero. The weighted average assumptions used to value the Company’s stock-based awards for the years ended December 31, 2015, 2014 and 2013, was as follows:
|  | Year Ended December 31, |
|  | 2015 |  |  | 2014 |  |  | 2013 |
Stock Options: |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.66 |  |  |  | 5.71 |  |  |  | 5.59 |
Estimated volatility |  |  | 56 | % |  |  | 61 | % |  |  | 60 | %
Risk-free interest rate |  |  | 1.55 | % |  |  | 1.73 | % |  |  | 0.87 | %
Expected dividend yield |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
Employee Stock Purchase Plan Rights: |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 0.75 |  |  |  | 0.76 |  |  |  | 0.50 |
Estimated volatility |  |  | 53 | % |  |  | 52 | % |  |  | 39 | %
Risk-free interest rate |  |  | 0.28 | % |  |  | 0.10 | % |  |  | 0.10 | %
Expected dividend yield |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
The weighted average grant-date fair value of stock options granted during the years ended December 31, 2015, 2014 and 2013, was $2.35 per share, $3.28 per share and $2.03 per share, respectively. The weighted average grant-date fair value of employee stock purchase rights during the years ended December 31, 2015, 2014 and 2013, was $1.54 per share, $1.42 per share and $1.18 per share, respectively.

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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 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 Reference 2: 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=51659978&loc=d3e25284-112666 Reference 3: 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 4: 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 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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