Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 3,990	$ 4,390
Accrued external research and development expenses	3,803	4,428
Accrued professional fees	2,070	1,759
Current portion of operating lease liabilities	826	844
Other	264	208
Accrued expenses and other current liabilities	$ 10,953	$ 11,629