This peptide is a HCV protease substrate incorporating an ester bond between residues P1 and P1. Due to ready transesterification of the scissile bond to the acyl-enzyme intermediate, this substrate shows very high kcat/Km values, enabling detection of activity with subnanomolar nonstructural protein 3 (NS3 protease) concentrations. It is widely used for the continuous assay of NS3 protease activity. Substrate cleavage is proportional to the enzyme concentration with a detection limit for NS3 between 1 nM and 250 pM. Upon cleavage of this substrate, fluorescence can be monitored at Abs/Em = 355/500 nm.