We assessed the CYP2D6 genotype and measured tamoxifen metabolites to determine their association with (1) menopausal symptoms, (2) liver mediated biomarkers of drug estrogenicity associated with breast cancer recurrence, including IGF-I, SHBG, and C-reactive protein (CRP)8, and (3) risk of recurrence. The gene discussed is CYP2D6; the disease is breast cancer.