Thus, we selected SULT1A1 and UGT1A1 as candidate genes for affecting breast tumor characteristics based on the following criteria: both genes are important effectors of the biological fate of estrogens (especially pharmacological estrogens), both genes are polymorphic, and functionally significant genotype–phenotype relationships for the SULT1A1 and UGT1A1 alleles have been well characterized. This evidence concerns the gene SULT1A1 and breast neoplasm.