The microbiome in periodontal disease is distinct from that seen in periodontal health, with an increased abundance of pathogenic bacterial communities.32,33Porphyromonas gingivalis is a key component of the so-called red complex of gram-negative bacteria associated with periodontal disease.9 It has been hypothesized that P gingivalis, through posttranslational citrullination of periodontal mucosal proteins, may provide an antigenic source for ACPA in RA.11 A recent study34 showed oral priming with P gingivalis can trigger an erosive ACPA-positive arthritis in an in vivo animal model. The gene discussed is PRTN3; the disease is rheumatoid arthritis.