In the present study, we investigated the pharmacokinetic profile of tamoxifen and its active metabolites in breast cancer patients, and explored the impact of CYP2D6, CYP2C9, CYP2C19, CYP3A5, POR, ABCB1 and UGT2B15 genotypes on endoxifen and MRE/NDM in Ethiopians, a population with high frequency distribution of active CYP2D6 gene duplication or multiplication. The gene discussed is CYP2D6; the disease is breast cancer.