11. Stock-Based Compensation

        The Company's results of operations include expenses relating to employee and non-employee stock-based awards as follows (in thousands):

                                                                                                                                                                                    

Valuation Assumptions

        The Company estimated the fair value of employee stock options using the Black-Scholes valuation model. The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period of the awards. The fair value of employee stock options were estimated using the following assumptions for the years ended December 31, 2016, 2015 and 2014:

                                                                                                                                                                                    

        Risk-free Interest Rate:    The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant.

        Volatility:    The Company used an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry using an average of historical volatilities of the Company's industry peers.

        Expected Term:    The Company uses the simplified method prescribed in the ASC 718, Compensation—Stock Compensation, to calculate the expected term of options granted to employees and directors.

        Expected Dividends:    The Company has not paid and does not anticipate paying any dividends in the near future.

        At December 31, 2016 and 2015, the unrecognized compensation expense associated with respect to options granted to employees was $18.5 million and $4.7 million, respectively, and is expected to be recognized on a straight-line basis over 3.28 and 3.64 years, respectively.

        Stock-based compensation expense related to awards to non-employees is recognized based on the then-current fair value at each measurement date over the associated service period of the award, which is generally the vesting term, on a straight line basis. The Company used the Black-Scholes valuation model to assist it in determining the fair value of stock-based awards. Stock-based compensation expense for non-employees was $142,500 and $42,848 for the years ended December 31, 2016 and 2015, respectively.

        The following assumptions were used in valuation of non-employee stock options: