Fibroblast Growth Factors （FGFs） comprise a family of at least eighteen structurally realted proteins that are involved in a multitude of physiological and pathological cellular processes， including cell growth， differentation， angiogenesis， wound healing and tumorgenesis. The biological activities of the FGFs are mediated by a family if type I transmembrane tyrosine kinases which undergo dimerization and autophosphorylation after ligand binding. Four distinct genes encoding closely related FGF receptors， FGFR-1to -4 are known. Multiple forms of FGFR-1 to -3 are generated by alternative splicing of the mRNAs. A frequent splicing event involving FGFR-1 and -2 results in receptors containing all three Ig domains， referred to as the alpha isoform， or only IgII and IgIII， referred to as the ? isoform. Only the alpha isoform has been identified for FGFR-3 and FGFR-4. Additional splicing events for FGFR-1 to -3， involving the C-terminal half of the IgIII domain encoded by two mutually exclusive alternative exons， generate FGF receptors with alternative IgIII domains （IIIb and IIIc）. A IIIa isoform which is a secreted FGF binding protein containing only the N-terminal half of the IgIII domain plus some intron sequences has also been reported for FGFR-1. Mutations in FGFR-1 to -3 have been found in patients with birth defects involving craniosynostosis.
