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Accrued Expenses
3 Months Ended
Mar. 31, 2021
Payables and Accruals [Abstract]  
Accrued Expenses Accrued Expenses
Accrued expenses consist of the following (in thousands):
 March 31,December 31,
 20212020
Payroll and employee related expenses$1,840 $3,049 
Collaboration and licensing1,350 1,350 
Accrued patent fees471 534 
Accrued external research and development costs2,010 2,029 
Accrued professional and consulting services802 798 
Accrued interest170 170 
Other204 216 
     Accrued expenses$6,847 $8,146