ESR1 and breast cancer: Breast cancer classification and stage evaluation have traditionally been based on clinicopathological features (patient age, lymph node invasion, tumor size, histological type, and pathology grade)3, but recent technological advances have made it possible to differentiate breast cancers by molecular markers such as ER (Estrogen Receptor), PR (Progesterone Receptor) and HER2 (Human epidermalgrowth factor receptor-2)4, 5.