The Fibroblast Growth Factor Receptor (FGFR) family is made up of four known closely related receptor tyrosine kinases, FGFR1 through 4. For FGFR1 through 3, alternative splicing results in numerous variants. One common splicing event results in the extracellular domain having all 3 immunoglobulin-like domains (alpha isoform) or only the II and III Ig domains (beta isoform). Additional variation in the IgIII domain allows for expression as IIIb or IIIc, again due to alternative splicing. Ligand binding results in dimerisation and autophosphorylation of specific tyrosine residues. Variants may exhibit different ligand binding properties. FGFR1 predominantly binds the FGF-acidic and FGF-basic proteins.