ESR1 and breast cancer: Based on the presence or absence of hormone receptors (estrogen receptor/ER, or progesterone receptor/PR), and excess levels of human epidermal growth factor receptor 2 (HER2+/HER2-), breast cancer is stratified into four major molecular subtypes, including luminal A (ER+ and/or PR+/HER2-), luminal B (ER+ and/or PR+/HER2+), HER2 overexpressing (ER-/PR-/HER2+), and basal-like or triple negative (ER-/PR-/HER2-) [10, 11].