ESR1 and breast cancer: Based on immunohistochemistry classification, breast cancer is classified to five major molecular subtypes: luminal A (estrogen receptor (ER)+, progesterone receptor (PR)+, human epidermal growth factor receptor 2 (HER2)−, Ki-67 low), luminal B HER2− (ER+, PR+, HER2−, Ki-67 high), luminal B HER2+ (ER+, PR+, HER2+, Ki-67 high), HER2 (ER−, PR−, HER2+), and basal-like (triple-negative (TNBC), ER−, PR−, HER2−), which are related to the clinical outcomes [3,4].