ErbB2 (also known as HER2/Neu) is the most potent oncogene among the epidermal growth factor receptor family. Although ErbB2 does not bind a ligand, it is constantly prone to dimerization and recruits the largest group of phosphotyrosine-binding proteins among all ERBB members. The ErbB2 gene is located on chromosome 17 (C17) and is amplified and overexpressed in about 20–30% of cases of primary human breast cancer, correlating with adverse patient outcome. ErbB2 overexpression impacts on various cancer cell-associated phenotypes including resistance to apoptosis, hyperproliferation, increased motility and constitutive activation of the PI3K/AKT pathway. Moreover, ErbB2- containing heterodimers are the most potent ErbB complexes as they show higher growth factor affinities, undergo slow endocytosis and are constantly re-cycled to the cell membrane. On the other hand, ErbB2 function has also been implicated in physiological processes such as heart or oligodendrocyte development.