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Note 6 - Stock-based Compensation (Tables)
 6 Months Ended
Jun. 30, 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    
706,004
     $
2.00
      
 
      
 
  
Exercised    
(74,001
 )   $
1.45
      
 
      
 
  
Forfeited    
(14,687
 )   $
16.92
      
 
      
 
  
Outstanding at June 30, 2017    
2,783,570
     $
7.03
      
7.3
     $
45
  
Exercisable at June 30, 2017    
1,388,426
     $
7.46
      
5.9
     $
34
  
Vested or expected to vest at June 30, 2017 (1)    
2,783,570
     $
7.03
      
7.3
     $
45
  
Common Stock Reserved for Future Issuance [Table Text Block]
    June 30,
2017		   December 31,
2016
         
Stock-based compensation awards    
3,572,457
      
2,982,470
  
Employee Stock Purchase Plan    
247,774
      
292,821
  
Total    
3,820,231
      
3,275,291