Of the five human PAD isoforms, increased levels of PAD2 and PAD4 are found in localised pathologic sites of RA and periodontitis (joints and periodontal pockets), leading to increased protein citrullination [39], and, as mentioned before, P. gingivalis (a key periodontitis pathogen) secretes a PAD called PPAD [40–42] as a virulence factor. This evidence concerns the gene PADI4 and rheumatoid arthritis.