In enzymology, a protein-arginine deiminase is an enzyme that catalyzes a form of post translational modification called arginine de-imination or citrullination. his chemical reaction induces citrullinated proteins formation with altered structure and function, leading to numerous pathological diseases, including inflammation and autoimmune diseases. These pathologies established the PADs as therapeutic targets and multiple PAD inhibitors are known. Humans encode five PADs, designated PADs 1-4 and PAD6. Of the five PAD isozymes (PAD1, 2, 3, 4 and 6), only four (PADs1-4) are catalytically active. PAD activity is tightly regulated by Ca2+ and PADs contain 4 (PAD1), 5 (PAD3, 4) or 6 (PAD2) Ca2+-binding sites. Dysregulated PAD activity, most notably PAD2 and PAD4, is associated with multiple inflammatory diseases (e.g., rheumatoid arthritis) as well as cancer, and PAD inhibitors, such as Cl-amidine and BB-Cl-amidine, show efficacy in multiple preclinical animal models of disease.