ErbB2, also called Neu and Her2, is a transmembrane glycoprotein in the ErbB family of tyrosine kinase receptors for EGF superfamily growth factors. ErbB2 is widely expressed in epithelial cells and over-expressed in a large number of breast carcinomas. ErbB2 has no identified ligands but heterodimerizes with ErbB1/EGF R, ErbB3, or ErbB4 to form higher affinity signaling complexes. The protease ADAM10 releases a 110 kDa soluble fragment of ErbB2 from the cell surface. ErbB2 plays roles in development, cancer, communication at the neuromuscular junction, and regulation of cell growth and differentiation. The ErbB2/ErbB3 heterodimer is expressed in the majority of breast, skin, ovary and gastrointestinal tumors and transduces a highly mitogenic signal in response to neuregulin 1 (NRG1; heuregulin 1) or NRG2. ErbB3, ErbB2 and neuregulin are all required for formation of the sympathetic nervous system.