Pg. possesses some virulence factors of special interest in the context of RA: it has its own citrullinating enzyme, Porphyromonas peptidylarginine deiminase (PPAD), which is expressed on the outer membrane of Pg. and differs from human PAD's in its Ca2+ independent enzymatic activity (62). This evidence concerns the gene PADI4 and rheumatoid arthritis.