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Note 6 - Stock-based Compensation (Tables)
 3 Months Ended
Mar. 31, 2017
Notes Tables  
Share-based Compensation, Stock Options, Activity [Table Text Block]
    Options   Weighted-
Average
Exercise
Price		   Weighted-
Average
Remaining
Contractual
Term (years)		   Aggregate
Intrinsic
Value
                 
Outstanding at December 31, 2016    
2,166,254
     $
8.55
      
7.1
     $
124
  
Granted    
659,342
     $
2.05
      
 
      
 
  
Exercised    
(74,001
 )   $
1.45
      
 
      
 
  
Outstanding at March 31, 2017    
2,751,595
     $
7.18
      
7.6
     $
64
  
Exercisable at March 31, 2017    
1,242,299
     $
7.35
      
6.0
     $
64
  
Vested or expected to vest at March 31, 2017 (1)    
2,751,595
     $
7.18
      
7.6
     $
64
  
Common Stock Reserved for Future Issuance [Table Text Block]
    March 31,
2017		   December, 31
2016
         
Stock-based compensation awards    
3,572,611
      
2,982,470
  
Employee Stock Purchase Plan    
292,821
      
292,821
  
Total    
3,865,432
      
3,275,291