USP1(Ubiquitin carboxyl-terminal hydrolase 1) is a negative regulator of DNA damage repair which specifically deubiquitinates monoubiquitinated FANCD2.It is Also involved in PCNA-mediated translesion synthesis (TLS) by deubiquitinating monoubiquitinated PCNA.This protein,which consists of 785 amino acids with a deduced molecular mass of 88.2 kDa,possesses His and Cys domains that are highly conserved in all members of the ubiquitin-specific processing (UBP) family of proteases. The USP1 enzyme can undergo autocleavage,resulting in a 100 kDa N-terminal fragment and a 14 kDa C-terminal fragment.