ESR1 and breast cancer: Breast cancer is a heterogeneous disease that can be classified, based on hormone and human epidermal growth factor receptor 2 (HER2) status, into three major subtypes: luminal (expressing the estrogen receptor (ER) and/or progesterone receptor (PR)), that can be further divided in luminal A (ER+/PR+/HER2−) and luminal B (ER+/PR+/HER2+); HER2-overexpressing (ER−/PR−/HER2+); and triple-negative (TNBC), lacking expression of ER, PR and HER-2 [1].