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Prepaid Expenses and Other Current Assets (Tables)
6 Months Ended
Jun. 30, 2017
Prepaid Expenses And Other Current Assets [Abstract]  
Schedule of prepaid expenses and other current assets
  December 31,  June 30, 
  2016  2017 
Research and development tax credit receivable $1,730  $603 
Prepayments  867   1,080 
Accounts receivable  10   89 
VAT receivable  327   356 
Deposits  132   132 
Other current assets  31   200 
  $3,097  $2,460