Accrued Expenses and Other Liabilities (Details) £ in Thousands, $ in Thousands	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2018 GBP (£)
Payables and Accruals [Abstract]
Compensation and benefits	$ 8,833	$ 8,732
Research and development costs	11,373	15,343
UCLB milestone and option	0	205	£ 350
Professional fees	2,289	3,005
Other liabilities	678	496
Total accrued expenses and other liabilities	$ 23,173	$ 27,781

Accrued License Fees, Current

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Accrued Research And Development Cost, Current

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Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Carrying value as of the balance sheet date of obligations incurred through that date and payable for professional fees, such as for legal and accounting services received. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Total of the carrying values as of the balance sheet date of obligations incurred through that date and payable for obligations related to services received from employees, such as accrued salaries and bonuses, payroll taxes and fringe benefits. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Amount of liabilities classified as other, due within one year or the normal operating cycle, if longer.

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