The neu gene was first identified in rat tumors that had been induced by the carcinogen ethyl nitrosourea. The human homolog of neu, usually designated c-erbB-2, is located on chromosome 17, q21. It specifies a transmembrane receptor-like phosphoglycoprotein that is closely related to the EGFr (c-erbB-1). The ligand for c-erbB-2 is not known. A significant proportion of adenocarcinomas (especially of the breast, colon, and pancreas) have amplification and/or overexpression of c-erbB-2. The unique qualities associated with the subset of tumors that overexpress c-erbB-2 have not yet been firmly identified.