FGF activity is mediated by a family of type I transmembrane tyrosine kinases, which undergo dimerization and autophosphorylation after ligand binding. Five distinct genes encode closely related FGF receptors, FGF R1 through 5. FGF Rs contain three Ig-like domains and a stretch of acidic residues between the first and second Ig-like domains. FGF R1, 2, 3, and -4 have a cytoplasmic split tyrosine-kinase domain, but FGF R5 does not. Multiple forms of FGF R1, 2, and 3 are generated by alternative splicing.