Thioglycolate-induced peritoneal macrophages from wild-type, PAD2-/-, and PAD4-/- mice or human peripheral blood-derived M1 macrophages were activated with a variety of stimulants, then fixed and stained with DAPI and either anti-citrullinated histone H4 (citH4) antibody or sera from ACPA+ or ACPA- rheumatoid arthritis subjects. This evidence concerns the gene PADI2 and rheumatoid arthritis.