This is the first study showing the influence of hormonal status on the pharmacokinetics of TAM and its metabolites END, 4-HTAM and NDTAM in pre- and postmenopausal TAM-treated breast cancer patients previously phenotyped as NMs for CYP2D6 and with in vivo CYP3A activity based on midazolam oral clearance [23–25]. The gene discussed is CYP3A4; the disease is breast cancer.