ESR1 and breast cancer: Human epidermal growth factor receptor 2/neu (HER2/neu) expression and hormone receptor status, including estrogen receptor (ER) and progesterone receptor (PR), are widely employed as biomarkers for breast cancer and are analyzed using immunohistochemistry (IHC) [3].These biomarkers are used as prognostic indicators to predict response to hormonal and chemotherapeutic treatments and in choosing adjuvant therapy for breast cancer.