Stockholders' Equity | Stockholders' Equity Convertible Preferred Stock As of September 30, 2017 and December 31, 2016, the Company had 5,000,000 shares of convertible preferred stock with a par value of $0.001 per share authorized and no preferred stock issued and outstanding. Warrants As of September 30, 2017 and December 31, 2016 , the Company had outstanding warrants to purchase 41,595 and 61,595 shares of common stock, respectively. In July 2017, a warrant to purchase 20,000 shares of common stock was net exercised by Essex Capital for 9,388 shares of common stock at an exercise price per share of $14.40 in accordance with the terms of the warrant agreement. Stock Option Plan 2014 Equity Incentive Plan and 2014 Inducement Plan On January 1, 2017, the number of shares of common stock reserved for issuance under the Company’s 2014 Equity Incentive Plan, or 2014 EIP, automatically increased by 4% of the total number of shares of the Company’s common stock outstanding on December 31, 2016, or 1,145,958 shares. During the nine months ended September 30, 2017 , the Company granted stock options for 868,625 shares of common stock and 310,575 restricted stock awards under the 2014 EIP. As of September 30, 2017 , there were 911,380 shares available for issuance under the 2014 EIP. During the nine months ended September 30, 2017 , the Company granted stock options for 35,000 shares of common stock and 95,000 restricted stock awards granted under the 2014 Inducement Plan (the "2014 IN"). As of September 30, 2017 , there were 288,867 shares available for issuance under the 2014 IN. The grant-date fair value of the employee stock options under the 2014 EIP and 2014 IN was estimated using the Black-Scholes option-pricing model with the following weighted-average assumptions: Three Months Ended Nine Months Ended 2017 2016 2017 2016 Expected term (in years) 6.0 6.0 6.0 6.0 Expected volatility 66.6 % 64.2 % 67.8 % 61.6 % Risk-free interest rate 2.0 % 1.2 % 2.1 % 1.4 % Expected dividend rate — % — % — % — % Fair Value of Common Stock . The fair value of the shares of common stock is based on the Company's stock price as quoted by the NASDAQ. Expected Term . The expected term for employees and non-employee directors is based on the simplified method, as the Company’s stock options have the following characteristics: (i) granted at-the-money; (ii) exercisability is conditioned upon service through the vesting date; (iii) termination of service prior to vesting results in forfeiture; (iv) limited exercise period following termination of service; and (v) options are non-transferable and non-hedgeable, or “plain vanilla” options, and the Company has a limited history of exercise data. The expected term for non-employee consultants is based on the remaining contractual term. Expected Volatility . As of January 1, 2017, the expected volatility is based on the historical volatility of a group of similar entities combined with the historical volatility of the Company, whereas prior to 2017, the expected volatility was based solely on the historical volatility of a group of similar entities. In evaluating similarity, the Company considered factors such as industry, stage of life cycle, capital structure, and size. Risk-Free Interest Rate . The risk-free interest rate is based on U.S. Treasury constant maturity rates with remaining terms similar to the expected term of the options. Expected Dividend Rate . The Company has not and does not plan to pay dividends in the foreseeable future, and therefore used an expected dividend rate of zero percent in the valuation model. Forfeitures. As of January 1, 2017, the Company adopted the forfeiture rate methodology change in accordance with ASC 2016-09 to account for forfeitures as they occur (Note 2). Prior to the adoption of ASC 2016-09, the Company was required to estimate forfeitures at the time of grant and revised those estimates in subsequent periods if actual forfeitures differed from those estimates. The Company used historical data to estimate pre-vesting option forfeitures and record stock-based compensation expense only for those awards that were expected to vest. To the extent actual forfeitures differed from the estimates, the difference was recorded as a cumulative adjustment in the period that the estimates were revised. In June 2017, an employee converted to a non-employee consultant and the individual's options and awards continued to vest in accordance with the 2014 EIP. There were no stock option grants made to non-employee consultants during 2017. The fair value of the stock options outstanding for non-employee consultants is calculated at each reporting date using the Black-Scholes option pricing model with the following weighted-average assumptions: Three Months Ended September 30, Nine Months Ended 2017 2016 2017 2016 Expected term (in years) 8.9 7.2 8.9 7.5 Expected volatility 68.1 % 66.5 % 68.6 % 69.7 % Risk-free interest rate 2.2 % 1.4 % 2.2 % 1.5 % Expected dividend rate — % — % — % — % 2014 Employee Stock Purchase Plan On January 1, 2017, the number of shares of common stock reserved for issuance under the Company’s 2014 Employee Stock Purchase Plan, or 2014 ESPP, automatically increased by 1% of the total number of shares of the Company’s common stock outstanding on December 31, 2016 , or 286,489 shares. As of September 30, 2017 , there were 931,181 shares available for issuance under the 2014 ESPP. The fair value of the option component of the shares purchased under the 2014 ESPP was estimated using the Black-Scholes option-pricing model with the following weighted-average assumptions: Three Months Ended Nine Months Ended 2017 2016 2017 2016 Expected term (in years) 0.5 0.5 0.5 0.5 Expected volatility 46.6 % 80.9 % 59.1 % 72.0 % Risk-free interest rate 1.1 % 0.4 % 0.9 % 0.4 % Expected dividend rate — % — % — % — % Fair Value of Common Stock . The fair value of the shares of common stock is based on the Company’s stock price. Expected Term . The expected term is based on the term of the purchase period under the 2014 ESPP. Expected Volatility . As of January 1, 2017 the expected volatility is based on the historical volatility of the Company's common stock. Prior to January 1, 2017, the expected volatility was based on volatility of a group of similar entities. In evaluating similarity, the Company considered factors such as industry, stage of life cycle, capital structure, and size. Risk-Free Interest Rate . The risk-free interest rate is based on U.S. Treasury constant maturity rates with remaining terms similar to the expected term. Expected Dividend Rate . The Company has never paid dividends and does not plan to pay dividends in the foreseeable future, and therefore used an expected dividend rate of zero percent in the valuation model. Total Stock-Based Compensation Total stock-based compensation expense related to options, restricted stock awards, and ESPP for employees, non-employee directors, and non-employee consultants was allocated as follows (in thousands): Three Months Ended Nine Months Ended 2017 2016 2017 2016 Research and development $ 1,534 $ 1,124 $ 4,341 $ 4,325 General and administrative 1,612 1,631 5,479 4,659 Total stock-based compensation expense $ 3,146 $ 2,755 $ 9,820 $ 8,984 |