ESR1 and breast cancer: The main breast cancer molecular subtypes are termed Luminal A (ER+/PR+/HER2-/low Ki-67); Luminal B (ER+/PR+/HER2-/+/high Ki-67); HER2-overexpression (ER-/PR-/HER2+) and basal/triple negative breast cancers (TNBCs) (ER-/PR-/HER2-) [4].