In April 2018, we sold 6.9 million shares of our common stock at a public offering price of $26.00 per share. We received total net proceeds of $168.7 million (net of $10.7 million in issuance costs) from the sale of the common stock.

In June 2018, we sold 5.1 million shares of our common stock at a public offering price of $39.50 per share. We received total net proceeds of $194.4 million (net of $5.6 million in issuance costs) from the sale of the common stock.

The following table summarizes the stock option activity for the three months ended September 30, 2019:

	Shares (in thousands)			Weighted- average Exercise Price		Weighted- average Remaining Contractual Term (Years)
Balance at December 31, 2018		15,265		$	18.33		6.87
Granted		985		$	21.18
Exercised		(1,793	)	$	11.29
Expired and forfeited		(1,378	)	$	22.65
Balance at September 30, 2019		13,079		$	19.06		7.34

For the nine months ended September 30, 2019, 1,793,000 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $20.2 million.

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	4,365	$	3,212	$	14,123	$	9,363
General and administrative		2,939		2,366		10,333		6,950
Sales and marketing		2,400		2,472		15,856		7,259
Total stock-based compensation expense	$	9,704	$	8,050	$	40,312	$	23,572

As of September 30, 2019, there was $80.2 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.6 years.

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:

	September 30,
	2019			2018
Risk-free interest rate		2.1	%		2.8	%
Dividend yield		0.0	%		0.0	%
Volatility		67.4	%		70.5	%
Expected life (years)		6			6

We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model. There were no new offering periods for the three months ended September 30, 2019 and 2018.