Figure 1  Subsite nomenclature for proteolytic enzymes is shown. Amino acid residues to the left of the polypeptide scissile amide bond are numbered sequentially, beginning with P1 and increasing toward the N-terminus. Amino acid residues to the right of the scissile bond are numbered sequentially, beginning with P1′ and increasing toward the C-terminus. Complimentary regions of the protease active site employ the corresponding S numbering.