ESR1 and breast cancer: Most basal-like breast cancers lack or show low levels of ER and PGR, and lack HER2 protein overexpression and HER2 gene amplification, while expressing genes and proteins typically found in “basal” or myoepithelial cells of the normal breast, such as high-molecular-weight cytokeratins (5/6, 14 and 17) [26,28,30,38], P-cadherin [39], caveolins [40,41], and EGFR [26], and, in a minority of cases, harboring EGFR gene amplification [42] or aneusomy [43].