ERBB2 belongs to the protein kinase superfamily, Tyr protein kinase family and EGF receptor subfamily. It contains a protein kinase domain. ERBB2 is widely expressed in epithelial cells, and amplification and/or overexpression of ErbB2 has been reported associated with malignancy and a poor prognosis in numerous carcinomas, including breast, prostate and ovarian cancers. Rat ERBB2 is an essential component of a neuregulin-receptor complex, although neuregulins do not interact with it alone. ErbB2 mediates signalling pathways which involve mitogen-activated protein kinase and phosphatidylinositol-3 kinase, this receptor plays a key role in development, cell proliferation and differentiation.