As of September 30, 2019, the aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2015 Equity Incentive Plan, or 2015 Plan, was 18,500,000 shares. The 2015 Plan also provides for the grant of nonstatutory stock options, or NSOs, incentive stock options, or ISOs, stock appreciation rights, restricted stock awards and restricted stock unit awards. At September 30, 2019, a total of 10,135,657 options were outstanding, of which 3,335,196 were exercisable, 616,643 restricted stock unit awards were outstanding, and 3,596,826 shares were available for future grant under the 2015 Plan.

On January 1, 2019, 500,000 shares of common stock were added to the 2017 Employee Stock Purchase Plan, or 2017 ESPP. In accordance with the provisions of our 2017 ESPP, the number of shares of our common stock reserved for issuance under the 2017 ESPP automatically increases on January 1^st of each calendar year. Under the ESPP, participating employees can elect to have a portion of their base pay withheld during a consecutive payment period for the purchase of shares of our common stock. At the conclusion of each offering period, participating employees can purchase shares of our common stock at 85% of the lesser of the closing price at the beginning or at the end of the period. As of September 30, 2019, the aggregate number of shares of common stock reserved under the 2017 ESPP was 1,500,000 and we had 1,428,725 shares available for future issuance under the 2017 ESPP. During the three and nine months ended September 30, 2019, 21,984 shares and 39,724 shares, respectively, were issued under our 2017 ESPP. At September 30, 2019, accrued liabilities included $0.2 million of 2017 ESPP contributions for which the related shares we anticipate issuing in the first quarter of 2020.

We measure stock-based compensation expense for equity-classified awards, principally related to stock options, restricted stock units, or RSUs, and stock purchase rights under our 2017 ESPP, based on the estimated fair value of the award on the date of grant. We recognize the value of the portion of the award that we ultimately expect to vest as stock-based compensation expense over the requisite service period in our condensed consolidated statements of operations. We reduce stock-based compensation expense for estimated forfeitures at the time of grant and revise in subsequent periods if actual forfeitures differ from those estimates.

We use the Black-Scholes model to estimate the fair value of stock options granted and stock purchase rights under our 2017 ESPP. The expected term of stock options granted represents the period of time that we expect them to be outstanding. As we do not have sufficient historical information, we use the simplified method for estimating the expected term. Under the simplified method we calculate the expected term as the average time-to-vesting and the contractual life of the options. As we gain additional historical information, we will transition to calculating our expected term based on our exercise patterns. For the nine months ended September 30, 2019 and 2018, we used the following weighted-average assumptions in our Black-Scholes calculations:

	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate		2.3	%		2.7	%
Dividend yield		0.0	%		0.0	%
Volatility		75.6	%		77.1	%
Expected life	6.1 years			6.1 years

Board of Directors Stock Options:

	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate		1.8	%		2.9	%
Dividend yield		0.0	%		0.0	%
Volatility		73.8	%		78.2	%
Expected life	6.3 years			6.4 years

	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate		2.4	%		1.9	%
Dividend yield		0.0	%		0.0	%
Volatility		60.0	%		64.2	%
Expected life	6 months			6 months

The following table summarizes stock-based compensation expense for the three and nine months ended September 30, 2019 and 2018 (in thousands):

	Three Months Ended September 30,					Nine Months Ended September 30,
	2019			2018		2019		2018
Cost of sales - product	$	127		$	—	$	382	$	—
Research and development expenses		2,814			2,313		9,298		6,868
Selling, general and administrative expenses		(6,405	)		10,417		19,779		24,371
Total	$	(3,464	)	$	12,730	$	29,459	$	31,239

As of September 30, 2019, total unrecognized, estimated non-cash stock-based compensation expense related to non-vested stock options and RSUs was $72.8 million and $14.8 million, respectively. We will adjust total unrecognized compensation cost for future forfeitures. We expect to recognize the cost of non-cash stock-based compensation expense related to non-vested stock options and RSUs over a weighted average amortization period of 1.4 years and 2.5 years, respectively.

On September 18, 2019, Paula Soteropoulos, former Chief Executive Officer, Sarah Boyce, former President, and Jeffrey M. Goldberg, former Chief Operating Officer entered into Separation Agreements that terminated their employment with us and, with respect to Ms. Soteropoulos and Ms. Boyce, their positions on our board of directors, effective September 18, 2019. The unvested equity awards for Ms. Boyce were terminated on the separation date. In addition, on September 18, 2019, we entered into a Consulting Agreement with Ms. Soteropoulos, which was amended on October 30, 2019, and a Consulting Agreement with Mr. Goldberg, pursuant to which they agreed to provide consulting services to us through November 4, 2019 and October 31, 2019, respectively. Under the terms of the 2015 Plan, their equity awards continued to vest through the end of each such consulting periods. We have updated our estimates of forfeitures for equity awards for these departed officers to reflect only those awards expected to vest, resulting in a reversal in the quarter ended September 30, 2019 of $18.8 million of stock-based compensation expense previously recognized.