Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 1,737	$ 4,390
Accrued external research and development expenses	5,239	4,428
Accrued professional fees	1,905	1,759
Current portion of operating lease liabilities	708	844
Other	127	208
Accrued expenses and other current liabilities	$ 9,716	$ 11,629