Nonstructural protein 3 (NS3), also known as p-70, is a viral nonstructural protein that is the 70 kDa cleavage product of the hepatitis C virus polyprotein. It acts as a serine protease. The C-terminal two-thirds of the protein also acts as a helicase and nucleoside triphosphatase. The first (N-terminal) 180 amino acids of NS3 have an additional role as cofactor domains for the NS2 protein.