ESR1 and breast cancer: Molecular profiling classified breast cancer into the following “intrinsic subtypes of breast cancer”: Normal-like, Luminal A (ER+ PR+/− HER2−), Luminal B (ER+ PR+/− HER2+ and/or Ki67+), HER2-enriched (PR+, and over-expression of HER2 due to a genomic amplification ERBB2) with around half of HER2-enriched tumors expressing ERα, and basal-like (ER− PR− HER2− and expressing basal cytokeratins, triple negative, TNBC) [2,3,4,5].