Breast cancer is a complex and heterogeneous disease that can be classified into at least four molecular subtypes: HER2 (human epidermal growth factor receptor 2), basal, luminal A and luminal B [17–19]: Based on transcriptional analyses of several breast cancer cell lines, Neve and his colleagues further classified breast cancers cell lines into 2 major clusters. The gene discussed is ERBB2; the disease is breast carcinoma.