ERBB2 and breast cancer: Breast cancer can be classified into different molecular subtypes according to gene expression: luminal A (Hormone Receptor positive (HR+)/human epidermal growth factor receptor 2 negative (HER2-), with high HR expression and low levels of the protein Ki-67), luminal B (HR+ either HER2+ or HER2- with high levels of Ki-67 and with low HR expression), HER2-enriched (amplification ofERBB2 gene, regardless of HR status) and triple-negative (HR-/HER2-).