ESR1 and breast cancer: Since Carey et al[5] demonstrated that immunohistochemistry (IHC) can effectively substitute gene expression-based molecular subtyping, several IHC markers, particularly the human epidermal growth factor receptor 2 (HER2), estrogen receptor (ER), progesterone receptor (PR), and Ki67, have been extensively utilized for classifying the molecular subtype of BC in clinical settings, offering the advantages of convenience, affordability, high sensitivity, and high specificity.