Even though these polymorphisms are well characterized in different populations, the frequency of CYP3A4*1B, CYP3A5*3 and UGT1A4*2 allele yet have not been determined in the population of Croatia (CRO), therefore the objectives of this study were to analyze the frequency of these SNPs and its possible association with anastrozole therapy-induced undesirable side effects among Croatian BC population. This evidence concerns the gene UGT1A4 and breast cancer.