ESR1 and triple-negative breast carcinoma: Based on histopathological criteria including the expression of hormone receptors (estrogen receptor and/or progesterone receptor) and/or human epidermal growth factor receptor 2 (HER2), four main subtypes of BC were distinguished, including luminal A, luminal B, HER2 positive (HER2+) and basal-like or triple-negative breast cancer (TNBC) [2].