Peptidyl arginine deiminase 4 (PAD4) activity is detected in K/BxN arthritic legs in wild type (WT) but not PAD4 knockout (KO) mice. PAD4 activity was measured by immunohistochemical detection of citrullinated histone H4 (H4Cit3). WT and PAD4 KO mice were immunized with pooled K/BxN serum as described. Mice were sacrificed 10 days post injection (p.i.) and their legs prepared for histology. Sections were blocked in goat serum and probed with either anti-H4Cit3 antibody or isotype control. Images were taken at 80× magnification. Specific staining of citrullinated histone H4 was apparent on WT but not PAD4 KO slides and is representative of at least 4 independent experiments.