ESR1 and breast cancer: The most widely used classification system is the one developed by the Cancer Genome Atlas (TCGA) network [2], which includes the following subtypes: luminal-like tumours, which consist of luminal-A, estrogen receptor +/progesterone receptor +/human epidermal growth factor receptor 2−/(ER+/PR+/HER2−) and luminal-B (ER+/PR−/HER2+) tumours, HER2-enriched breast cancers, which exhibit overexpression of the receptor tyrosine-protein kinase Erb-B2 oncogene, and basal-like tumours, that are heterogeneous and a more aggressive subtype [2,3].