ESR1 and breast cancer: According to the reported study, molecular subtypes of BC can be classified into luminal A (ER+/PR+/HER2-/low Ki-67), luminal B (ER+/PR+/HER2-/+/high Ki-67), HER2-overexpression (ER-/PR-/HER2+), and triple negative breast cancers/TNBCs (ER-/PR-/HER2-) [15].