Protein-arginine deiminase type-4 (PADI4) catalyzes the citrullination/deimination of arginine residues of proteins. PADI4 down-regulates histone H3 and H4 arginine methylation, both by preventing arginine methylation by CARM1 and HRMT1L2/PRMT1 and by converting methylarginine to citrulline. PADI4 is e, Xenopus/Amphibian,pressed in eosinophils and neutrophils but is not e, Xenopus/Amphibian,pressed in peripheral monocytes or lymphocytes. PADI4 is a rheumatoid arthritis susceptibility locus due to its important role in increasing citrullination of proteins in rheumatoid arthritis synovial tissues leading to a break in tolerance to citrullinated peptides processed and presented in the appropriate HLA conte, Xenopus/Amphibian,t.