Although the exact pathogenic effect of smoking on RA still remains uncertain, several mechanisms have been proposed to better understand how cigarette smoking plays a role in various autoimmune diseases [3,4,5,6,7,8], and citrullination has been reported to be an important factor for the development of RA in the anti-citrullinated protein antibody (ACPA)-positive subset. The gene discussed is PRTN3; the disease is rheumatoid arthritis.