In December 2017, the Company adopted the 2017 Equity Incentive Plan (the “2017 Plan”), which initially reserved approximately 6.4 million shares of common stock for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. The 2017 Plan provides that the number of shares reserved and available for issuance under the 2017 Plan will automatically increase each January 1, beginning on January 1, 2019, by the least of (i) 10.0 million shares, (ii) 5% of the outstanding shares on the last day of the immediately preceding fiscal year, or (iii) such number of shares determined by the administrator of the 2017 Plan. In January 2019, common stock available for issuance under the 2017 Plan was increased by approximately 4.8 million shares as a result of this automatic increase provision.

Awards granted under the 2017 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value of the Company's common stock on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.

In May 2015, the Company adopted the 2015 Stock Incentive Plan (the “2015 Plan”), which as amended, reserved approximately 8.3 million shares of common stock for the issuance of stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants under terms and provisions established by the Board of Directors and approved by the stockholders. Awards granted under the 2015 Plan expire no later than ten years from the date of grant. For stock options, the option price shall not be less than 100% of the estimated fair value of the Company's common stock on the day of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.

Upon adoption of the 2017 Plan, no new awards or grants are permitted under the 2015 Plan, and the approximately 0.2 million shares that were then unissued and available for future award under the 2015 Plan became available under the 2017 Plan. The 2015 Plan will continue to govern restricted stock awards and option awards previously granted thereunder.


	Number of Options	Weighted- Average Exercise Price		Weighted- Average remaining contractual life (years)	Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2018	9,612,652	$	10.49	8.20	$	97,804
Options granted	3,610,835	18.58
Options exercised	(786,638)	4.27
Options forfeited	(796,115)	16.72
Balance at December 31, 2019	11,640,734	$	12.96	7.79	$	51,879
Options vested and expected to vest at December 31, 2019	9,896,002	$	15.13	8.17	$	22,672
Options exercisable at December 31, 2019	3,938,523	$	11.19	7.38	$	24,552

Aggregate intrinsic value represents the difference between the fair value of the Company's common stock and the exercise price of outstanding options. The total intrinsic value of options exercised was $13.1 million, $9.8 million and $3.5 million for the years ended December 31, 2019, 2018 and 2017, respectively. During the years ended December 31, 2019, 2018, and 2017 the weighted-average grant-date fair value of the options vested was $10.09, $3.85, and $1.94 per share, respectively. The weighted-average grant date fair value of all options granted during the years ended December 31, 2019, 2018 and 2017 was $11.86, $14.79, and $6.41 per share, respectively.


	Year Ended December 31,
	2019	2018	2017
Expected term (in years)	5.50 - 6.08	5.50 - 6.08	6.08
Volatility	65.5% - 77.8%	79.4% - 87.4%	81.9% - 91.3%
Risk-free interest rate	1.5% - 2.6%	2.6% - 3.1%	1.8% - 2.3%
Dividend yield	—	—	—

Expected Term: The expected term represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term).

Expected Volatility: The Company uses an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends as the Company does not have sufficient trading history for its common stock. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available.

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.

In August and November 2015, the Board of Directors granted approximately 1.6 million and 0.1 million shares of performance- and market- contingent awards to members of the senior management team, respectively. These awards have an exercise price of $0.68 per share.

These awards have two separate market triggers for vesting based upon either (i) the successful achievement of stepped target closing prices on a national securities exchange for 90 consecutive trading days later than 180 days after the Company’s initial public offering for its common stock, or (ii) stepped target prices for a change in control transaction. In the event that neither of these market triggers are achieved by the specified timelines, such awards will terminate with respect to that portion of the shares. The expense recognized associated with these performance- and market- contingent awards was $5.2 million and $0.5 million of general and administrative and research and development expense, respectively, during the year ended December 31, 2019. The remaining expense for these awards, to be recognized using the accelerated attribution method, is $0.3 million, which will be recognized over the remaining derived service period of approximately nine months.

The Company used a lattice model with a Monte Carlo simulation to value these stock options. This valuation methodology utilized the estimated fair value of the Company’s common stock on grant date and several key assumptions, including expected volatility of the Company’s stock price based on comparable public companies, risk-free rates of return and expected dividend yield.

Stock-based compensation related to stock options granted to non-employees is recognized as the stock options are earned. Prior to the adoption of ASU 2018-07 during the third quarter of 2018, the unvested options granted to non-employees were revalued using the Company's estimate of fair value on each reporting date. Subsequent to the adoption of ASU 2018-07, existing stock options granted to non-employees are no longer revalued, and the estimated fair value of new stock options granted to non-employees is calculated on the date of grant and not remeasured, similar to stock options granted to employees.

Under the 2017 Plan, the Company may grant restricted stock awards ("RSAs"), which represent restricted shares of issued common stock for which the recipient's rights in the stock are restricted until the shares are vested, and restricted stock units ("RSUs"), which represent a commitment to issue shares of common stock in the future upon vesting. The fair value of restricted stock underlying the RSAs and RSUs is determined based on the closing market price of the Company's common stock on the date of grant.


	Share Awards & Units	Weighted-Average Fair Value at Date of Grant per Share




Unvested at December 31, 2018	503,243	$	4.86
Granted	850,234	18.86
Vested and released	(419,064)	2.63
Forfeited	(51,777)	20.50
Unvested at December 31, 2019	882,636	$	18.67
Expected to vest – December 31, 2019	882,636	$	18.67

The aggregate intrinsic value of RSUs is calculated as the closing price per share of the Company's common stock on the last trading day of the fiscal period, multiplied by the number of RSUs expected to vest. The total intrinsic value of RSUs expected to vest was $15.4 million as of December 31, 2019. During the year ended December 31, 2018, the weighted-average grant-date fair value of RSUs granted was $15.92. No RSAs or RSUs were granted in the year ended December 31, 2017. The total fair value of RSAs and RSUs that vested during the years ended December 31, 2019, 2018, and 2017 (measured on the date of vesting) was $8.8 million, $37.0 million and $14.3 million, respectively. There was $13.7 million of total unrecognized stock-based compensation related to unvested RSUs at December 31, 2019, all of which is expected to be recognized over a remaining weighted-average period of 3.2 years.

In December 2017, the Company adopted the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which initially reserved 1.0 million shares of common stock for employee purchases under terms and provisions established by the Board of Directors. The 2017 ESPP provides that the number of shares reserved and available for issuance under the 2017 ESPP will automatically increase each January 1, beginning on January 1, 2019, by the least of (i) 2.0 million shares, (ii) 1% of the outstanding shares on the last day of the immediately preceding fiscal year, or (iii) such number of shares determined by the administrator of the 2017 Plan. In January 2019, common stock available for issuance under the 2017 ESPP was increased by approximately 1.0 million shares as a result of this automatic increase provision.

Under the 2017 ESPP, employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of common stock on the first trading day of each offering period or on the exercise date. The 2017 ESPP provides for consecutive, overlapping 12-month offering periods. The offering periods are scheduled to start on the first trading day on or after May 31 or November 30 of each year, except for the first offering period which commenced on December 8, 2017, the first trading day after the effective date of the Company’s registration statement. Contributions under the 2017 ESPP are limited to a maximum of 15% of an employee's eligible compensation. We issued 0.2 million and 0.1 million shares of common stock under the 2017 ESPP during the years ended December 31, 2019 and 2018, respectively.


	Year Ended December 31,
	2019	2018	2017
Expected term (in years)	0.50 - 1.00	0.50 - 1.00	0.48 - 0.98
Volatility	47.6% - 59.1%	61.0% - 64.3%	50.0% - 51.3%
Risk-free interest rate	1.6% - 2.4%	2.1% - 2.7%	1.5% - 1.7%
Dividend yield	—	—	—

As of December 31, 2019, total unamortized stock-based compensation expense related to employee and non-employee awards was $76.2 million. The weighted-average period over which such stock-based compensation expense will be recognized is approximately 2.7 years.