ESR1 and breast cancer: Currently, clinical practice classifies BC in four subtypes, based on the molecular expression of Estrogen Receptor (ER), Progesterone Receptor (PR), Human Epidermal growth factor Receptor 2 (HER2) and Ki67 proliferative marker: (i) Luminal A (ER+/PR+/HER2-/lowKi-67), (ii) Luminal B (ER+/PR+/HER2-/+/high Ki-67), (iii) HER2-overexpression (ER-/PR-/HER2+), and (iv) Triple Negative Breast Cancers/TNBCs (ER-/PR-/HER2-) [2].