ABSTRACT
A variety of hormones and growth factors have been implicated in the regulation of eukaryotic cell function. In general, hormones are defined as compounds that are released into the circulation under controlled conditions, and that act on target tissues that are distal to the site of origin of the hormones. Autocrine action is defined as the effects mediated by a particular compound that acts directly on its cell of origin rather than on a neighboring cell. It is generally accepted today that hormones may also act locally, and that many factors that exert autocrine effects are also acting in a paracrine fashion. The importance of the epidermal growth factor receptor in the regulation of cellular proliferation is underscored by the observation that it is coded for by a protooncogene, and that it exhibits a strong sequence homology with the protein product of the avian erythroblastosis virus v-erb B oncogene.
