ERBB3 and cancer: Despite their individual limitations, HER3 contributes synergistically to HER2-mediated cell transformation and amplifies malignant properties of a tumor driven by HER2 overexpression; indeed, the HER2-HER3 heterodimer is considered the most potent HER mitogenic complex, which functions as an oncogenic unit that activates the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) and mitogen-activated protein kinase (MAPK) pathways in cancer [10–12].