Porphyromonas gingivalis (Pg), a key pathogen driving periodontitis, has specifically been implicated in contributing to the development of anti-citrullinated protein antibody (ACPA)-positive RA due to its capacity to citrullinate proteins by expression of a peptidyl arginine deiminase (PAD) enzyme,3 4 and by triggering release of neutrophil extracellular traps containing citrullinated histones.5 The gene discussed is PRTN3; the disease is rheumatoid arthritis.