Secreted polypeptide growth factors such as insulin, insulinlike growth factor I (IGF-I), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and colony-stimulating factor (CSF-1) initiate complex cytoplasmic and nuclear events. These distinct cellular responses can be attributed to the interaction of hormones with their respective cell-surface receptors. The cytoplasmic domains of these receptors exhibit an unusual protein kinase that phosphorylates tyrosine residues (Ushiro and Cohen 1980; Ek et al. 1982; Kasuga et al. 1982; Rubin et al. 1982) and has led to their designation as members of a tyrosine kinase receptor family.