Proteins participating in the process of citrullination, such as the enzymes peptidylarginine deiminase 4 (PAD4) and the periodontitis-causing bacteria Porphyromonas gingivalis (Pg), can also trigger the production of autoantibodies (anti-PAD4 and anti-Pg) in RA patients. This evidence concerns the gene PADI4 and periodontitis.