PADI4 and Arthritis: It is clear that there is still much to be answered; however, our research, through a superficial study of microbial proteins, suggests that the homologs of PAD/AD and the microbial citrullinome are widely distributed in the human microbiome and not restricted to P. gingivalis and E. coli. And, even though it is likely that many species have the ability to generate citrullinated peptides, many of these modified peptides may be irrelevant for inducing arthritis.