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DIPF is a diagnostic test for identifying the active sites of serine protease by reacting it with DIPF, which results in irreversible inactivation of enzymes. DIPF does not react with other Ser residues, even those on the same protein. Chymotrypsin's Ser 195 is the only residue with which DIPF interacts, proving that this residue is the active site Ser for the enzyme.

Affinity labeling is done for the identification of active site residue of His-57. In this method, an analog of the substrate containing a reactive group specifically attaches to the enzyme's active site, where it interacts to establish a stable covalent bond with a neighboring susceptible group. It is then possible to separate and identify the group(s) using the affiliation label.

Hence, the DIPF test and affinity labeling are ways to identify amino acid residues.