Breast tumors are very heterogeneous and can be classified in three main groups based on their molecular profile: luminal cancers that express estrogen and/or progesterone receptors; human epidermal growth factor receptor 2 (HER2)-positive cancers that express the tyrosine kinase receptor ERBB2; and triple-negative (TN) cancers in which none of these receptors is detected. The gene discussed is ERBB2; the disease is breast neoplasm.