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 cash 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 cash 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 June 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		455		$	24.47
Exercised		(1,542	)	$	10.52
Expired and forfeited		(633	)	$	20.83
Balance at June 30, 2019		13,545		$	19.31		6.99

For the six months ended June 30, 2019, 1,542,000 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $16.2 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	4,434	$	3,115	$	9,758	$	6,151
General and administrative		3,714		2,308		7,394		4,584
Sales and marketing		4,558		2,398		13,456		4,787
Total stock-based compensation expense	$	12,706	$	7,821	$	30,608	$	15,522

As of June 30, 2019, there was $84.5 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 2.5 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:

	June 30,
	2019			2018
Risk-free interest rate		2.4	%		2.7	%
Dividend yield		0.0	%		0.0	%
Volatility		68.0	%		71.1	%
Expected life (years)		6			6

We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:

	June 30,
	2019			2018
Risk-free interest rate		2.4	%		2.1	%
Dividend yield		0.0	%		0.0	%
Volatility		58.4	%		59.8	%
Expected life (months)		6			6