Gene expression analysis using microarrays allowed for the classification of breast tumors into molecular subtypes which first displayed an obvious specificity in their gene expression markers and were then identified using immunohistochemistry; they include the luminal A, luminal B, Her2-positive, and triple-negative subtypes [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. The gene discussed is ERBB2; the disease is breast neoplasm.