Protein-Arginine Deiminase Type-4 (PADI4) belongs to the Protein Arginine Deiminase family. PADI4 is expressed in eosinophils and neutrophils, but it is not expressed in peripheral monocytes or lymphocytes. PADI4 catalyzes the citrullination/deimination of arginine residues of proteins. In addition, PADI4 may play an important role in tumourigenesis. Genetic variations in PADI4 are a cause of susceptibility to rheumatoid arthritis (RA), which is a systemic inflammatory disease with autoimmune features and a complex genetic component.