ESR1 and breast cancer: According to current international guidelines, BC is classified into several subtypes based on the molecular profiling of the estrogen receptor (ER), the progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), including hormone receptor-positive (HR-positive)/HER2-negative, HR-positive/HER2-positive, HR-negative/HER2-positive, and triple-negative breast cancer (TNBC) [2].