Four distinct genes encoding closely related FGF receptors FGFR1-4 are known All four genes for FGFRs encode proteins with an N-terminal signal peptide three immunoglobulin (Ig)-like domains an acid-box region containing a run of acidic residues between the IgI and IgII domains a transmembrane domain and the split tyrosine-kinase domain Multiple forms of FGFR1-3 are generated by alternative splicing of the mRNAs A frequent splicing event involving FGFR1 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 beta isoform