The action of transforming growth factor peptides is mediated by distinct membrane receptors, which in turn activate a postreceptor signaling mechanism, eventually resulting in a mitogenic response. Such a signaling pathway may be modified by oncogene expression at the receptor or postreceptor levels, as well as by alterations in the levels of expression of the growth factor itself. One of the most extensively studied group of receptors is the type I epidermal growth factor receptor family. Activation of this receptor triggers the induction of receptor dimerization, which enables cross-phosphorylation to occur between two receptor molecules. This dimerization model provides a universal mechanism to activate the type I receptor family for growth factors and subsequent transformation.