11. Stock-based Compensation

In February 2016, the Company’s Board of Directors approved an amendment to the 2010 Long Term Incentive and Stock Award Plan (“2010 Plan”) to, among other things, allow for an increase in the number of shares of common stock reserved for issuance and recommended that the amendment be submitted to the Company’s shareholders for approval at the 2016 annual meeting. At the May 2016 annual meeting, the shareholders approved an amendment to the 2010 Plan to, among other things, increase the aggregate number of shares authorized for issuance by 3,400,000 shares to an aggregate of 22,171,206 shares.

The Company grants qualified and non-qualified stock options, RSUs, common stock and other stock-based awards under various plans to directors, officers, employees and other individuals. Stock options are granted at exercise prices of not less than the fair market value of the Company’s common stock on the date of grant. Additionally, the Company has an Employee Stock Purchase Plan (“ESPP”) that allows employees to purchase Company shares at a purchase price equal to 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last day of the offering period.

Stock Options

The stock options generally vest monthly over four years for employees and one year for directors. Stock options held by employees who qualify for retirement age (defined as employees that are a minimum of 55 years of age and the sum of their age plus years of full-time employment with the Company exceeds 70 years) vest on the earlier of scheduled vest date or the date of retirement.  The fair value of the stock options granted during the three and six months ended June 30, 2016 and 2015, was estimated based on the following weighted average assumptions:

Stock option activity for the six months ended June 30, 2016, was as follows:

Restricted Stock Units

RSUs generally vest over three years for employees and one year for directors.  In February 2016, the Company granted 2.3 million RSUs to employees that will vest one year from the date of grant. RSUs held by employees who qualify for retirement age (defined as employees that are a minimum of 55 years of  age and the sum of their age plus years of full-time employment with the Company exceeds 70 years) vest on the earlier of scheduled vest date or the date of retirement.

The valuation of RSUs is determined at the date of grant using the closing stock price.

Unvested RSU activity for the six months ended June 30, 2016, is summarized below:

Stock-based Compensation Expense

The following table shows total stock-based compensation expense for stock options, RSUs and ESPP in the condensed consolidated statements of comprehensive loss (in thousands):