One of the first studies that investigated the polymorphisms in CYP2D6 and SULT1A1 in 226 BC patients on tamoxifen, presented with differences in recurrence of the disease based on the homozygosity/heterozygosity of the CYP2D6*4 and SULT1A1*1 alleles (410). This evidence concerns the gene CYP2D6 and breast cancer.