The activity of fibroblast growth factors(FGFs)is mediated by a dual-receptor system. This comprises a family of four receptor tyrosine kinases(FGFRs)and heparin sulphate proteoglycans(HSPG). The binding of the FGFs to the FGFRSs is marked by a pattern of overlapping specificity despite alternative splicing events generating a large number of FGFRs.HSPG receptors may stimulate the combination between the FGFs and FGFRs, and provide additional specifity allowing a cell to fine tune its response to the FGFs.FGFRs induce the activities of donwstream signal molecules which then act in different ways that affect the cell's development, mitogenesis, and neuronal differentation, respectivly.