The discovery of a dominantly transforming oncogene in ENU-induced rat CNS tumours has revealed in rats and man a gene highly related to the EGF receptor. The gene may be activated experimentally by mutation or overexpression such that its expression can convert immortalised but non-tumourigenic rat fibroblasts to a tumourigenic state. The mechanism by which this activation occurs is not known, nor is it known whether the molecule is itself a growth factor receptor. It is hypothesised, however, that in the light of its similarity to the EGF receptor it may be involved in growth regulation and that activation may occur by producing an aberrant growth control signal. The gene has been found to be amplified frequently and the protein it encodes overexpressed in a variety of human adenocarcinomas. Investigations are underway to attempt to reveal its role in these tumours and to assess its possible value as an indicator of tumour behaviour. Overexpression of the protein provides a target for tumour imaging and therapy.