ESR1 and triple-negative breast carcinoma: According to the histological stratification, which is based on the expression levels of the progesterone receptor (PR), estrogen receptor (ER), and human epidermal growth factor receptor 2 (HER2), Breast cancer can be classified into distinct subtypes: normal-like, HER2-enriched, luminal A, luminal B, and triple-negative breast cancer (TNBC or basal-like breast cancer)2,3.