ESR1 and breast cancer: Breast cancer is highly heterogeneous and is clinically classified into several subtypes based on the expression of the estrogen receptor (ERα), PR (progesterone receptor), and human epidermal growth factor 2 (HER2): the luminal A subtype (ERα+ and/or PR+ with low levels of Ki-67), the luminal B subtype (ERα+ and/or PR+ with high levels of Ki-67), the HER2 subtype (HER2+), and triple-negative subtype (ER−/PR−/HER2−, TNBC)2,3.