Given the complexity of TAM metabolism and the scarce information on this topic in the literature, this descriptive study examines relationships between TAM glucuronide metabolite concentrations and genotypes for UGT1A4 Pro24Thr, UGT1A4 Leu48Val, UGT2B7 His268Tyr, UGT2B15 Asp85Tyr, UGT2B15 Lys523Thr, UGT2B17del in 132 Spanish patients with estrogen receptor-positive breast cancer under tamoxifen treatment. Here, UGT2B15 is linked to breast cancer.