Note 6 - Stock-based Compensation

6 Months Ended

Jun. 30, 2016

Notes to Financial Statements

Disclosure of Compensation Related Costs, Share-based Payments [Text Block]

6. Stock-based Compensation

Stock Options

The following table summarizes stock option activity for employees and non-employees:

	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2015		2,200,369		$	8.52		7.8	$	15,705
Granted		114,162		$	6.77
Exercised		(72,508	)	$	1.91
Cancelled or forfeited		(7,089	)	$	4.92
Outstanding at June 30, 2016		2,234,934		$	8.66		7.6	$	4,470
Exercisable at June 30, 2016		997,718		$	5.57		5.9	$	3,577
Vested or expected to vest at June 30, 2016 (1)		2,064,955		$	8.41		7.4	$	4,373

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(1) Represents the number of vested options at June 30, 2016 plus the number of unvested options expected to vest based on the unvested options outstanding at June 30, 2016 .

Employee Stock Purchase Plan

The 2014 ESPP initially authorized the issuance of up to 140,500 shares of Common Stock. The number of shares increases each January 1, commencing on January 1, 2015 and ending on (and including) January 1, 2024, by an amount equal to the lesser of one percent of the outstanding shares as of the end of the immediately preceding fiscal year, 281,000 shares, or any lower amount determined by the Company’s Board of Directors prior to each such January 1st. The Company’s Board of Directors has determined there was to be no increase on January 1, 2016. As of June 30, 2016, the 2014 ESPP authorized the issuance of up to 304,991 shares of Common Stock. The first offering under the 2014 ESPP began on January 1, 2015 and ended on June 30, 2015. During the six months ended June 30, 2016, 674 shares were issued under the 2014 ESPP with 296,685 shares remaining for future issuance under the plan as of June 30, 2016. The Company incurred $16,000 and $32,000 in stock-based compensation expense related to the 2014 ESPP for the three and six months ended June 30, 2016, respectively. The Company incurred $11,000 and $22,000 in stock-based compensation expense related to the 2014 ESPP for the three and six months ended June 30, 2015, respectively.

Common Stock

The Company has the following shares reserved for future issuance:

	June 30, 2016		December 31, 2015
Stock-based compensation awards		3,007,453		2,419,901
Employee Stock Purchase Plan		296,685		297,359
Total		3,304,138		2,717,260