ESR1 and triple-negative breast carcinoma: In accordance with different expression patterns, BC can be classified into four subtypes: luminal A (ER+, PR+, HER2−, low Ki-67 index), luminal B (ER+, PR+, HER2+ or HER2−, high Ki-67 index), HER2-positive (ER−, PR−, HER2+) and triple-negative breast cancer (TNBC, ER−, PR−, HER2−) [6].