Given the complexity of TAM metabolism and the inconsistent results provided in the literature, this descriptive study was designed to examine relationships between TAM metabolite concentrations and genotypes for CYP2D6, CYP3A4, CYP3A5, SULT1A1, SULT1A2 and SULT1E1 in 135 patients with estrogen receptor-positive breast cancer. This evidence concerns the gene CYP3A4 and breast cancer.