Giving some support for an association between smoking and leukemia, a study carried out in Japan showed an increased risk for leukemia when occurring a joint occupational exposure to benzene in the presence of the NQO1 enzyme homozygous polymorphism [odds ratio, OR = 7.6 (95% CI 1.8–31.2; Rothman et al., 1997)]. The gene discussed is NQO1; the disease is leukemia.