Diagnostic technology now exists to aid in determining which tamoxifen patients are potentially receiving suboptimal treatment from existing alterations in one or more of the approximately 80 alleles of the gene coding for the CPYP450 2D6 enzyme. Recognition of this phenomenon provides physicians and their patients the opportunity for considering the use of other anti-estrogen drugs such as aromatase inhibitors in women whose CYP450 2D6 phenotype puts them at risk for poor response to tamoxifen therapy.