ESR1 and breast cancer: Due to the significant prognostic and predictive consequence of such diversity, researchers around the world turned their attention towards refining the well accepted and highly annotated histopathological classification based on the expression of the well-known breast cancer molecular biomarkers Estrogen receptor (ER), Progesterone Receptor (PR), and Human Epidermal Growth Factor Receptor 2 (HER2) which guide treatment choices.