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