ESR1 and triple-negative breast carcinoma: Breast cancer is generally classified into four distinct molecular subtypes, namely luminal A, luminal B, HER2-positive, and triple-negative breast cancer (TNBC or basal-like), and are characterized by the lack or presence of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) [5].