In the present study, we investigated the pharmacokinetics profile and the influence of CYP2D6, CYP2C9, CYP2C19, CYP3A5, POR, ABCB1 and UGT2B15 polymorphisms on the plasma level of tamoxifen and its active metabolites in Ethiopian breast cancer patients receiving adjuvant tamoxifen. The gene discussed is CYP3A5; the disease is breast carcinoma.