Protein Arginine Deiminase 2 (PAD2) is a guanidino-modifying enzyme that catalyzes the conversion of specific arginine residues to citrulline in a calcium-dependent manner. This enzyme has been implicated in several diseases, including rheumatoid arthritis (RA), retinal degeneration, and certain cancers. PAD2 has been shown to modify vimentin, fibrinogen and B/y-actin. Extracellular levels of PAD2 are increased in the lungs of smokers, providing a link between smoking as a risk factor for RA and anti-citrullinated protein antibodies among RA patients. Cayman's PAD2 Inhibitor Screening Assay (Ammonia) provides a convenient method for screening human PAD2 inhibitors.