Finally, we did not consider tumor-associated determinants of intrinsic endocrine resistance (e.g., variable effects of endoxifen on blocking transcription of ER-target genes involved in proliferation and migration) or acquired resistance, yet neoadjuvant studies with pre- and post-treatment biopsies, as well as longitudinal studies to survey the emergence of resistant ESR1 mutations, may offer future strategies. Here, ESR1 is linked to neoplasm.