ESR1 and breast cancer: The gene expression profile revealed that the levels of Estrogen (ER), Progesterone (PR) hormone receptors (HR) and HER2 overexpression characterize tumors of different subtypes, including Luminal A (ER + and/or PR + and HER2-), Luminal B (ER + and/or PR + and HER2+), HER2 overexpressed (ER- PR- HER2+) and triple negative (TN; ER- PR- HER2-) breast cancer [2-5].