PADI4, also named PADI5 and PDI5, catalyzes the citrullination/deimination of arginine residues of proteins such as histones, thereby playing a key role in histone code and regulation of stem cell maintenance. PADI4 is expressed in hematologic and rheumatoid arthritis synovial tissues. PADI4 haplotype associated with susceptibility to rheumatoid arthritis increases the production of citrullinated peptides, leading, in a cytokine-rich milieu, to a break in tolerance to citrullinated peptides processed and presented in the appropriate HLA context. Immunohistochemistry detected significant PADI4 expression in various malignancies and PADI4 expression was not detected in non-tumor tissues. (PMID: 19183436)