ESR1 and breast cancer: BC is a heterogeneous disease that is classified to four main subtypes: luminal A (ER+ and/or PR+/HER2−/low Ki-67), luminal B (ER+ and/or PR+/HER2+/high Ki-67), non-luminal HER2+ (ER− and PR−/HER2 overexpressed), and triple negative BC (TNBC, ER−, PR−, HER2−) [3].