Purity >95%. Cl-amidine is an inhibitor of protein arginine deiminases (PAD; IC50 s = 0.8, 6.2, and 5.9 μM for PAD1, PAD3, and PAD4 in vitro, respectively). It irreversibly inactivates all four PAD subtypes (kinact /KI = 37,000, 1,200, 2,000, and 13,000 M-1 min-1 for PAD1-4, respectively) by covalently modifying an active site cysteine that is important for its catalytic activity. Cl-amidine is cytotoxic to HL-60, MCF7, and HT-29 cancer cell lines (IC50 s = 0.25, 0.05 and 1 μM, respectively). In a collagen-induced mouse model of inflammatory arthritis, Cl-amidine dose-dependently decreases the citrulline content in serum and joints and reduces the development of IgG autoantibodies.