ESR1 and breast carcinoma: The estrogen receptor (ER), progesterone receptor (PR), and erb-b2 receptor tyrosine kinase 2 (ERBB2, also known as human epidermal growth factor receptor 2 (HER2)), are representative molecular biomarkers that distinguish breast cancer subtypes, i.e., ER+/HER2−, ER+/HER2+, ER−/HER2+, and ER−/HER2− [2].