ESR1 and breast cancer: The use of common molecular subtypes of BC such as luminal A (ER+/PR+/HER2−), luminal B (ER+/PR−/HER2), HER2-positive (ER−/PR−/HER2+) and triple negative breast cancer (TNBC, ER−/PR−/HER2) has advanced patient management and standard treatments [3].