Peptidyl arginine deiminase 4 (PAD4) is present in the arthritic joint. PAD4 was detected in infiltrating cells (a), localised to the cytoplasm of mononuclear cells (c,e). Unimmunised animals were negative for PAD4 staining (f). Immunohistochemical stainings were performed with a rabbit anti-PAD4 antibody. Control staining was performed with preimmune rabbit sera (b,d) (Original magnifications: ×40 (a,b,f); ×200 (c-e)).