Polypeptide growth factors are relatively small and stable molecules that communicate short range signals for cell fate determination. Unlike steroid hormones, which penetrate through the plasma membrane due to their hydrophobic character, these factors must bind a cell surface-localized receptor. In contrast to multispan receptors for neuropeptides and chemical transmitters, growth factor receptors transverse the plasma membrane only once, and their cytoplasmic domains harbor a tyrosine-specific catalytic activity, called protein tyrosine kinase. Ligand binding to a receptor tyrosine kinase (RTK) initiates a cascade of biochemical and phenotypic events known as the pleiotropic response. In addition to ion channels and membrane enzymes, cytoskeletal rearrangements, cytoplasmic enzymes, and adhesive properties of the growth factor-stimulated cell are also modified within seconds to hours. This plethora of surface and cytoplasmic events culminates in the regulation of gene transcription, eventually leading to alterations in cellular behavior ranging from entry into the cell cycle to terminal differentiation (for a specific example of the pleiotropic response to a growth factor see ref. 1).