ESR1 and breast cancer: Four BC subtypes were determined according to the ER/PR/HER2 status: ER+/HER2− or PR+/HER2− (Luminal A), ER+/HER2+ or PR+/HER2+ (Luminal B), ER−/PR−/HER2+ (HER2 overexpressing), and ER−/PR−/HER2− [triple-negative breast cancer (TNBC)].