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Note 2 - Significant Accounting Policies (Tables)
3 Months Ended
Mar. 31, 2016
Notes Tables  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
    Three Months Ended
March 31,
    2016   2015
Research and development   $ 598     $ 456  
General and administrative     1,219       766  
Total share-based compensation expense   $ 1,817     $ 1,222  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
    Three Months Ended
March 31,
    2016   2015
Dividend yield     -       -  
Expected volatility   65%
-
68%     68%  
Risk-free interest rate  
1.31%
- 1.82%   1.39%
-
2.12%
Expected life (in years)   5
-
10   5
-
10
Weighted-average grant date fair value per share     $8.01       $9.56