CYP3A4 and breast cancer: Genetic variations in enzymes involved in steroidogenesis have been suggested to play a role not only in the risk of breast cancer, but also in the age at menarche.25 The association of earlier menarche with the presence of the CYP3A4*1B gene has been demonstrated in a study conducted with women from the United States (adjusted OR = 3.21; 95% CI: 1.62–6.89).25