Polymorphisms of peptidyl-arginine deiminase 4 (PADI4), one of the enzymes that catalyze the citrullination process, highly predispose male smokers to RA, and the genetic heterogeneity observed in the PADI4 polymorphism between populations of Asian and European countries may be partly explained by differences in smoking prevalences among men [23]. The gene discussed is PADI4; the disease is rheumatoid arthritis.