Specificity of numerous proteases is determined by enzyme-substrate interactions in an extended active site cleft. The Schechter and Berger nomenclature (1) designates residues carboxy-terminal to the scissile peptide bond as prime side (P’) and amino-terminal residues as non-prime side (P). P and P’ residues interact with complementary protease subsites called S and S’. While some proteases show very restrictive specificity profiles (e.g. trypsin is specific for lysine and arginine in P1), other proteases display broad specificity profiles involving multiple subsite preferences.