Background
Autoantibodies (Ab) directed against citrulline (Cit)-containing proteins have a specificity of nearly 100% in rheumatoid arthritis (RA) patients. The presence of these markers early in disease, even before clinical onset, and the observation that these autoantibodies are produced locally in the pannus suggest an involvement in the pathogenesis. The targeted epitopes are generated by deimination, a post-translational modification catalyzed by the enzyme peptidyl arginine deiminase.