The strongest difference between normal breast tissue and breast tumours was observed for HER3. Together with HER2, HER3 generates the most mitogenic dimer in the HER-family with the capacity to signal both through the mitogen-activated protein kinase (MAPK) pathway for cell proliferation and through the phosphatidylinositol-3 ́-kinase (PI3K)-Akt pathway for cell survival [46]. The gene discussed is ERBB3; the disease is breast neoplasm.