ESR1 and breast cancer: BC is commonly classified into three subtypes: luminal, characterized by high expression of estrogen receptors (ER) and often progesterone receptors (PR); HER2-positive (HER2+), marked by overexpression of the epidermal growth factor receptor 2 (HER2); and triple-negative breast cancer (TNBC), which lacks expression of ER, PR, and HER27.