Rheumatoid arthritis (RA) is characterized by synovial inflammation [1] as well as the production of autoantibodies such as rheumatoid factor and antibodies against self-proteins (anticitrullinated peptide antibodies [ACPA]) that underwent citrullination, a posttranslational modification generated by the peptidylarginine deiminase [2]. The gene discussed is PRTN3; the disease is rheumatoid arthritis.