Original articles published in the last two decades have been searched in PubMed, using the following search terms: “extracellular vesicles and breast cancer”, “extracellular vesicles and endocrine resistance”, “extracellular vesicles and tamoxifen resistance”, “extracellular vesicles and aromatase inhibitor resistance”, “extracellular vesicles and estrogen receptor down regulators”. This evidence concerns the gene ESR1 and breast cancer.