ESR1 and breast cancer: According to this classification, there are five subtypes of BC, namely Luminal A (ERα+ and PR+; HER2–; Ki67 low), luminal B/HER2- (ERα+ and PR+; HER2-; Ki67 high), luminal B/HER2+ (ERα+ and PR+; HER2+), HER2-enriched (ER–, PR–, HER2+), and triple-negative BC (ER–, PR–, HER2–) [4].