ErbB2 is a member of the receptor tyrosine kinase family of proteins. The four members of this family are ErbB1 (Epidermal growth factor receptor; EGFR/HER1), ErbB2 (HER2/Neu), ErbB3 (HER3), and ErbB4 (HER4). The ErbB receptor members are single-pass transmembrane proteins with an extracellular ligand-binding domain and a cytoplasmic tyrosine-kinase domain. The epidermal growth factor (EGF) and its related proteins serve as the ligands for the ErbB receptors. Upon binding, the receptors heterodimerize, autophosphorylate and make contact with adapter molecules to transmit cellular signals involved in cell proliferation, survival, and differentiation. The ErbB2 receptor is the only member that does not interact with a ligand, but instead functions as a heterodimeric partner to coordinate signals from other ligand bound ErbB receptors. Amplification and/or overexpression of ErbB2 have been reported in many cancers.