The recombinant HCV NS3/4A protease mutant A156S is derived from its wild type (NCBI Accession: CAB46913) with a substitution of Ala156 with Ser. This single mutation is reported to be responsible for the resistance to VX-950, a potent HCV NS3/4A protease inhibitor tested in phase I clinical trial. But this mutant remains sensitive to BILN2061, another potent HCV NS3/4A protease inhibitor. This mutated form is important to be tested when screening for new HCV NS3/4A protease inhibitors.