11. Stock-based Compensation

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 months ended March 31, 2017 and 2016, was estimated based on the following weighted average assumptions:

Stock option activity for the three months ended March 31, 2017, was as follows:

In February 2017, the Board of Directors approved a grant of 1,018,000 stock options to members of the board, executives, and non-executive employees, subject to approval by the Company’s stockholders of an increase in the available shares under the Amended and Restated 2010 Long Term Incentive and Stock Award Plan at the 2017 Annual Meeting of Stockholders.

Restricted Stock Units

RSUs generally vest annually over three years for employees and one year for directors. 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.

RSU activity for the three months ended March 31, 2017, 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):