HER2 activates intracellular signalling pathways, including rat sarcoma (RAS)/rapidly accelerated fibrosarcoma (RAF)/mitogen-activated protein kinase kinase (MEK)/mitogen-activated protein kinase (MAPK)/myelocytomatosis oncogene (MYC)/c-jun, HER2/EGFR, HER2/HER3, Akt-mammalian target of rapamycin (mTOR), phosphatidylinositol-3-kinase (PI3K), and cyclin-D-cyclin-dependent protein kinase (CDK) complexes (7, 15–21). The gene discussed is ERBB2; the disease is fibrosarcoma.