Hepatitis C virus NS3 protein is a serine protease which has a trypsin-like fold. The non-structural (NS) protein NS3 is one of the NS proteins involved in replication of the HCV genome. NS2-3 proteinase, a zinc-dependent enzyme, performs a single proteolytic cut to release the N-terminus of NS3. The action of NS3 proteinase (NS3P), which resides in the N-terminal one-third of the NS3 protein, then yields all remaining non-structural proteins. The C-terminal two-thirds of the NS3 protein contain a helicase. The functional relationship between the proteinase and helicase domains is unknown. NS3 has a structural zinc-binding site and requires cofactor NS4A.