Approximately 75% of diagnosed cases of BC present with estrogen receptors (ERs), and these cases are termed ER-positive (ER+) BC; adjuvant endocrine therapies, such as treatment with tamoxifen (TMX) and aromatase inhibitors (AIs), have shown great potential in terms of reducing the risk of BC recurrence and increasing the survival rate [4,5]. This evidence concerns the gene ESR1 and breast cancer.