Note 11 - Stockholders' Equity	12 Months Ended
Dec. 31, 2017
Notes to Financial Statements
Stockholders' Equity Note Disclosure [Text Block]	11. Stockholders ’ Equity Common Stock 201 6 ATM Agreement On June 21, 2016, the Company entered into a Controlled Equity Offering SM Sales Agreement, or the Sales Agreement, or 2016 ATM Agreement, with Cantor Fitzgerald & Co., or Cantor, as agent, pursuant to which the Company may offer and sell, from time to time through Cantor, shares of the Company’s common stock, or the Common Stock having an aggregate offering price of up to $40.0 million, or the Shares. The offering of Shares pursuant to the Sales Agreement will terminate upon the earlier of (a) the sale of all of the Shares subject to the Sales Agreement or (b) the termination of the Sales Agreement by Cantor or the Company, as permitted therein. The Company will pay Cantor a commission rate in the low single digits on the aggregate gross proceeds from each sale of Shares and have agreed to provide Cantor with customary indemnification and contribution rights. During the year ended December 31, 2017, the Company issued and sold 5.4 million shares of common stock pursuant to the 2016 ATM Agreement, for which the Company received net proceeds of approximately $15.7 million, after deducting commissions, fees and expenses of $0.5 million. Stock Plans 2006 Stock Plan In August 2006, the Company establi shed the 2006 Plan in which 342 shares of common stock were originally reserved for the issuance of incentive stock options, or ISOs, and nonstatutory stock options, or NSOs, to employees, directors or consultants of the Company. In February 2008, an additional 375 shares of common stock were reserved for issuance under the 2006 Plan and, in November 2009, an additional approximately 1.4 million shares of common stock were reserved for issuance under the 2006 Plan. Per the 2006 Plan, the exercise price of ISOs and NSOs granted to a stockholder who at the time of grant owns stock representing more than 10% of the voting power of all classes of the stock of the Company could not be less than 110% of the fair value per share of the underlying common stock on the date of grant. Effective upon the execution and delivery of the underwriting agreement for the Company’s IPO, no additional stock options or other stock awards may be granted under the 2006 Plan. 2011 Equity Incentive Plan In January 2011, the Board of Directors adopted, and the Company’s stockholders approved, the 2011 Equity Incentive Plan, or 2011 Incentive Plan, as a successor to the 2006 Plan. The 2011 Incentive Plan became effective immediately upon the execution and delivery of the underwriting agreement for the IPO on February 10, 2011. As of February 10, 2011, no more awards may be granted under the 2006 Plan, although all outstanding stock options and other stock awards previously granted under the 2006 Plan will continue to remain subject to the terms of the 2006 Plan. The approximately 52 shares reserved under the 2006 Plan that remained available for future grant at the time of the IPO were transferred to the share reserve of the 2011 Incentive Plan. The initial aggregate number of shares of the Company ’s common stock that may be issued pursuant to stock awards under the 2011 Incentive Plan is approximately 1.9 million shares, which number was the sum of (i) 52 shares remaining available for future grant under the 2006 Plan at the time of the execution and delivery of the underwriting agreement for the Company’s IPO, and (ii) an additional approximately 1.8 million new shares. Then, the number of shares of common stock reserved for issuance under the 2011 Incentive Plan will automatically increase on January 1st each year, starting on January 1, 2012 and continuing through January 1, 2020, by 4% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or such lesser number of shares of common stock as determined by the Board of Directors. The term of the option is determined by the Board of Directors on the date of grant but shall not be longer than 10 years. Options under the 2011 Equity Incentive Plan generally vest over four years, and all options expire after 10 years. The Company issues new shares for settlement of vested restricted stock units and exercises of stock options. The Company does not have a policy of purchasing its shares relating to its share-based programs. 2011 Employee Stock Purchase Plan Additionally, in January 2011, the Board of Directors adopted, and the Company’s stockholders approved, the 2011 Employee Stock Purchase Plan, or the ESPP, which also became effective immediately upon the execution and delivery of the underwriting agreement for the IPO. Initially, 250 shares of the Company ’s common stock were authorized for issuance under the ESPP pursuant to purchase rights granted to the Company’s employees or to employees of any of its designated affiliates. The number of shares of the Company’s common stock reserved for issuance will automatically increase on January 1st each year, starting January 1, 2012 and continuing through January 1, 2020, in an amount equal to the lower of ( 1 ) 2% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or ( 2 ) a number of shares of common stock as determined by the Board of Directors. If a purchase right granted under the ESPP terminates without having been exercised, the shares of the Company’s common stock not purchased under such purchase right will be available for issuance under the ESPP. As of December 31, 2017, 94,893 shares have been issued to employees and there are 1,041,249 shares available for issuance under the ESPP. The weighted average fair value of shares issued under the ESPP in 2017, 2016 and 2015 was $2.59, $2.98 and $4.48 per share, respectively.

11.

Stockholders

’ Equity

Common Stock

201

ATM Agreement

June 21, 2016,

the Company entered into a Controlled Equity Offering

Sales Agreement, or the Sales Agreement, or

2016

ATM Agreement, with Cantor Fitzgerald & Co., or Cantor, as agent, pursuant to which the Company

may

offer and sell, from time to time through Cantor, shares of the Company’s common stock, or the Common Stock having an aggregate offering price of up to

$40.0

million, or the Shares. The offering of Shares pursuant to the Sales Agreement will terminate upon the earlier of (a) the sale of all of the Shares subject to the Sales Agreement or (b) the termination of the Sales Agreement by Cantor or the Company, as permitted therein. The Company will pay Cantor a commission rate in the low single digits on the aggregate gross proceeds from each sale of Shares and have agreed to provide Cantor with customary indemnification and contribution rights. During the year ended

December 31, 2017,

the Company issued and sold

5.4

million shares of common stock pursuant to the

2016

ATM Agreement, for which the Company received net proceeds of approximately

$15.7

million, after deducting commissions, fees and expenses of

$0.5

million.

Stock Plans

2006

Stock Plan

August 2006,

the Company establi

shed the

2006

Plan in which

342

shares of common stock were originally reserved for the issuance of incentive stock options, or ISOs, and nonstatutory stock options, or NSOs, to employees, directors or consultants of the Company. In

February 2008,

an additional

375

shares of common stock were reserved for issuance under the

2006

Plan and, in

November 2009,

an additional approximately

1.4

million shares of common stock were reserved for issuance under the

2006

Plan. Per the

2006

Plan, the exercise price of ISOs and NSOs granted to a stockholder who at the time of grant owns stock representing more than

10%

of the voting power of all classes of the stock of the Company could

not

be less than

110%

of the fair value per share of the underlying common stock on the date of grant. Effective upon the execution and delivery of the underwriting agreement for the Company’s IPO,

additional stock options or other stock awards

may

be granted under the

2006

Plan.

2011

Equity Incentive Plan

January 2011,

the

Board of Directors adopted, and the Company’s stockholders approved, the

2011

Equity Incentive Plan, or

2011

Incentive Plan, as a successor to the

2006

Plan. The

2011

Incentive Plan became effective immediately upon the execution and delivery of the underwriting agreement for the IPO on

February

10,

2011.

As of

February

10,

2011,

more awards

may

be granted under the

2006

Plan, although all outstanding stock options and other stock awards previously granted under the

2006

Plan will continue to remain subject to the terms of the

2006

Plan. The approximately

shares reserved under the

2006

Plan that remained available for future grant at the time of the IPO were transferred to the share reserve of the

2011

Incentive Plan.

The initial aggregate number of shares of the Company

’s common stock that

may

be issued pursuant to stock awards under the

2011

Incentive Plan is approximately

1.9

million shares, which number was the sum of (i)

shares remaining available for future grant under the

2006

Plan at the time of the execution and delivery of the underwriting agreement for the Company’s IPO, and (ii) an additional approximately

1.8

million new shares. Then, the number of shares of common stock reserved for issuance under the

2011

Incentive Plan will automatically increase on

January

1st

each year, starting on

January

2012

and continuing through

January

2020,

of the total number of shares of the Company’s common stock outstanding on

December

of the preceding calendar year, or such lesser number of shares of common stock as determined by the Board of Directors. The term of the option is determined by the Board of Directors on the date of grant but shall

not

be longer than

years. Options under the

2011

Equity Incentive Plan generally vest over

four

years, and all options expire after

years. The Company issues new shares for settlement of vested restricted stock units and exercises of stock options. The Company does

not

have a policy of purchasing its shares relating to its share-based programs.

2011

Employee Stock Purchase Plan

Additionally, in

January 2011,

the

Board of Directors adopted, and the Company’s stockholders approved, the

2011

Employee Stock Purchase Plan, or the ESPP, which also became effective immediately upon the execution and delivery of the underwriting agreement for the IPO.

Initially,

250

shares of the Company

’s common stock were authorized for issuance under the ESPP pursuant to purchase rights granted to the Company’s employees or to employees of any of its designated affiliates. The number of shares of the Company’s common stock reserved for issuance will automatically increase on

January

1st

each year, starting

January

2012

and continuing through

January

2020,

in an amount equal to the lower of (

)

of the total number of shares of the Company’s common stock outstanding on

December

of the preceding calendar year, or (

) a number of shares of common stock as determined by the Board of Directors. If a purchase right granted under the ESPP terminates without having been exercised, the shares of the Company’s common stock

not

purchased under such purchase right will be available for issuance under the ESPP.

As of

December

31,

2017,

94,893

shares have been issued to employees and there are

1,041,249

shares available for issuance under the ESPP. The weighted average fair value of shares issued under the ESPP in

2017,

2016

and

2015

was

$2.59,

$2.98

and

$4.48

per share, respectively.