ESR1 and breast cancer: The molecular subtype of TNBC, accounting for approximately 10–15% of breast cancer with a classification based on absence of estrogen receptor (ER), progesterone receptor, and human epidermal growth factor receptor 2 (HER2) markers, is characterized by a more aggressive course of the disease and limited treatment options as compared to the hormone receptor-positive luminal and HER2-amplified breast cancer types [19].