Breast cancer is a complex and highly heterogeneous disease classified on the basis of global gene expression analyses into at least five biologically different intrinsic subtypes (i.e. luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)-enriched, basal-like, and normal-like) with distinct morphologic features, variable clinical outcomes and disparate therapeutic responses [229]. The gene discussed is ERBB2; the disease is breast carcinoma.