ESR1 and breast cancer: To distinguish the heterogeneity of this disease, breast cancer has been categorized as four distinct subtypes based on gene expression profiling, including luminal A (ER-positive and/or PR-positive, HER2-negative, low Ki67 index), luminal B (ER-positive and/or PR-positive, HER2-positive or HER2 negative, higher Ki67 index), HER2 enriched (HER2-positive and ER-negative/PR-negative) and triple-negative/basal like (ER-negative, PR-negative and HER2-negative) [1], [25]–[27],[28].