As of June 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 June 30, 2019, a total of 13,068,974 options were outstanding, of which 5,382,040 were exercisable, 32,669 restricted stock unit awards were outstanding, and 3,824,563 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, 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 increased on January 1^st of each calendar year. As of June 30, 2019, the aggregate number of shares of common stock reserved under the 2017 ESPP was 1,500,000 and we had 1,450,709 shares available for future issuance under the 2017 ESPP. During the three and six months ended June 30, 2019, no shares and 17,740 shares, respectively, were issued under our 2017 ESPP. At June 30, 2019, accrued liabilities included $0.5 million of 2017 ESPP contributions for which the related shares were issued on July 1, 2019.

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 six months ended June 30, 2019 and 2018, we used the following weighted-average assumptions in our Black-Scholes calculations:

	Six Months Ended June 30,
	2019			2018
Risk-free interest rate		2.5	%		2.8	%
Dividend yield		0.0	%		0.0	%
Volatility		76.3	%		77.7	%
Expected life	6.1 years			6.1 years

Board of Directors Stock Options:

	Six Months Ended June 30,
	2019			2018
Risk-free interest rate		1.9	%		2.9	%
Dividend yield		0.0	%		0.0	%
Volatility		74.3	%		78.2	%
Expected life	6.3 years			6.4 years

	Six Months Ended June 30,
	2019			2018
Risk-free interest rate		2.5	%		1.6	%
Dividend yield		0.0	%		0.0	%
Volatility		64.1	%		62.3	%
Expected life	6 months			6 months

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Cost of sales - product	$	137	$	—	$	255	$	—
Research and development expenses		2,563		2,242		6,484		4,555
Selling, general and administrative expenses		11,663		9,884		26,184		13,954
Total	$	14,363	$	12,126	$	32,923	$	18,509

As of June 30, 2019, total unrecognized, estimated non-cash stock-based compensation expense related to non-vested stock options and RSUs was $82.1 million and $0.9 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 1.46 years and 1.45 years, respectively.