Tamoxifen is one of the most successful and widely used chemopreventive agents ever, and is an effective therapeutic agent
for inhibiting growth of hormone receptor positive breast cancers. Tamoxifen and some of its metabolites bind to estrogen
receptors and allow subsequent DNA binding at estrogen responsive genes, blocking some estrogenic signals while maintaining
others, depending on the tissue. When used therapeutically for up to five years, cases of tamoxifen resistance appear, requiring
alternative therapies. One recent proposal uniquely targets a zinc finger of the DNA binding domain of estrogen receptors,
rather than the ligand binding domain, to circumvent resistance. In light of the most recent clinical data, however, it is
now clear that aromatase inhibitors are the preferred first line therapy for all stages of breast cancer in post-menopausal
women, whether they have had previous tamoxifen exposure or resistance.