Dysregulation of signaling pathways, such as TGF-β, Wnt, Hedgehog, Notch, IL-6, Integrins, VEGF, HER2, EGFR, PI3K/Akt, and MAPK, leads to breast cancer cell proliferation and metastasis, as depicted in Figure 1. The gene discussed is ERBB2; the disease is breast cancer.