ESR1 and breast carcinoma: Breast cancer is the most common cancer in women worldwide, and the risk increases with age, reproductive factors, personal or family history of breast disease, genetic predisposition, and environmental factors.31 Two key target molecules have been identified in breast cancer pathogenesis namely ERα, which is expressed in approximately 70% of invasive breast cancers, and epidermal growth factor 2 (formerly HER2 or HER2/neu), which is overexpressed in approximately 20% of breast cancers.