Gene expression profiling has resulted in the classification of breast cancer into five molecular subtypes: Luminal A (ER+, PR+ and Her2−), luminal B (ER+, PR+ and Her2+), basal-like (ER−, PR− and Her2−; triple-negative) Her2-enriched (ER−/PR−/Her2+) and the normal breast-like subtype. This evidence concerns the gene ESR1 and breast cancer.