Based on the expression of estrogen receptors (ERs), progesterone receptors (PRs), human epidermal growth factor receptor 2 (HER2), and the measurement of tumor proliferation rate through the Ki67 level, breast cancer can be further classified into luminal A (ER- or PR-positive, or both; HER2-negative, low proliferation), luminal B (ER- or PR-positive, or both; HER2-negative, high proliferation), HER2-overexpressing, or triple-negative (ER, PR, and HER2-negative) [16]. This evidence concerns the gene ESR1 and breast cancer.