FIBROBLAST GROWTH FACTORS are a family of seven structurally related polypeptides characterized by high affinity to heparin. They are highly mitogenic for cells derived from mesoderm and neuroectoderm and are among the most potent inducers of the formation of mesenchyme and new blood vessels-~. This gene family includes the prototypes acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) which, unlike most other polypeptide growth factors, are primarily cell-associated and lack a conventional signal sequence for secretion-:~. Studies with neutralizin?, anti-bFGF antibodies demonstrate, however, the occurrence of an extracellular autocrine activity of bFGF:~-5, suggesting that FGF may be released from cells via an unusual secretory route that does not require a signal peptide. A family of high affinity ceil-surface FGF receptors with intrinsic tyrosine kinase activities has been identified, as have low affinity receptors that consist of heparan sulfate proteoglycans (HSPGs) I-:~. Binding of FGF to the high affinity receptors leads to stimulation of protein kinase activity and activation of early gene transcription. However, the relationship of these events to FGF-stimulated cell proliferation remains unclear.