ESR1 and breast neoplasm: Microarray-based gene expression profiling of breast tumors has identified at least six major intrinsic subtypes—luminal A, luminal B, human epidermal growth factor receptor 2-positive/estrogen receptor-negative (HER2+/ER−), basal-like, claudin-low, and normal-like—that are thought to originate from different precursor cells and follow different progression pathways [4]-[6].