Citrullinated proteins are present in the arthritic joint. Positive citrulline staining was found in extracellular deposits (a,e), cartilage (c) and infiltrating cells (d). Unimmunised animals were negative for citrulline (f). Immunohistochemical staining was performed with RA3, a human recombinant anti-citrullinated protein antibody. Control staining was performed with an isotyped-matched recombinant human anti-U1-70K antibody (b). (Original magnifications: ×100 (a,b); ×250 (c); ×400 (d); ×40 (e,f)).