Based on an epidemiological link between PD and ACPA+ RA, and the unique feature of the periodontal pathogen Porphyromonas gingivalis to generate citrullinated proteins, it has been hypothesised that break of tolerance to citrullinated proteins and production of ACPA may occur in the gum mucosa (7, 14, 20). Here, PRTN3 is linked to rheumatoid arthritis.