Trypsin （EC3.4.21.4） is part of the serine protease family. Trypsin cleaves lysine and arginine at the C-terminal side of the peptide. The hydrolysis rate is slower if an acidic residue is on either sides of the cleavage site and no cleavage occurs if a proline residue is on the carboxyl side of the cleavage site. The stringent specificity of trypsin is essential for protein identification, and it has become the gold standard for protein digestion to peptides for shotgun proteomics. Trypsin optimum pH is pH-7 to 9. Trypsin will also hydrolyze ester and amide linkages of synthetic derivatives of amino acids such as: benzoyl L-arginine ethyl ester （BAEE）, p-toluenesulfonyl- L-arginine methyl ester （TAME）, tosyl-L-arginine methyl ester, N-α-benzoyl-L-arginine p-nitroanilide （BAPNA）, L-lysyl-p-nitroanilide, and benzoyl-L-tyrosine ethyl ester （BTEE）. Serine protease inhibitors that inhibit recombinant trypsin include TLCK （N-p-tosyl-L-lysine chloromethyl ketone）, PMSF （phenylmethanesulfonyl fluoride）, benzamidine, soybean trypsin inhibitor, and ovomucoid.