11.

Stockholders’ Equity

Common Stock Offering

January 2017,

we sold

14.1

million shares of our common stock at a public offering price of

$12.20

per share. We received total net proceeds of

$163.7

million (net of

$8.8

million in issuance costs) from the sale of the common stock.

December 2017,

we sold

9.7

million shares of our common stock at a public offering price of

$15.50

per share. We received total net proceeds of

$142.6

million (net of

$7.4

million in issuance costs) from the sale of the common stock.

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.

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.

Stock Option

Activity

The following table summarizes the stock option activity for the

nine

months ended

September

30,

2018:

						Weighted-
						a verage
				Weighted-		Remaining
				a verage		Contractual
	Shares			Exercise		Term
	(in thousands)			Price		(Years)
Balance at December 31, 2017		13,463		$	15.03		8.01
Granted		725		$	32.42
Exercised		(1,332	)	$	11.84
Expired and forfeited		(556	)	$	17.94
Balance at September 30, 2018		12,300		$	16.27		7.50

For the

nine

months ended

September 30, 2018,

1,332,000

shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of

$15.8

million.

Stock-based Compensation

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,
	2018		2017		2018		2017

Research and development	$	3,212	$	2,595	$	9,363	$	8,758
General and administrative		2,366		2,419		6,950		7,345
Sales and marketing		2,472		2,462		7,259		7,544
Total stock-based compensation expense	$	8,050	$	7,476	$	23,572	$	23,647

As of

September 30, 2018,

there was

$73.8

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:

	September 30 ,
	20 18			20 17
Risk-free interest rate		2.8	%		2.0	%
Dividend yield		0.0	%		0.0	%
Volatility		70.5	%		80.2	%
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. There were

new offering periods for the

three

months ended

September 30, 2018

and

2017.