Breast cancer can be classified according to its molecular subtype, and hormonal receptors status: estrogen receptor positive (ER+) and/or progesterone receptor positive (PR+); Human Epidermal Growth Factor Receptor-2 (HER2) positive (HER2+); or triple negative (ER−, PR−, HER2−) breast cancer (TNBC) [7]. This evidence concerns the gene ESR1 and breast cancer.