 The natural alternate splicing of four fibroblast growth factor receptor (FGFR) genes results in the production of over 48 different isoforms of FGFR.These isoforms vary in their ligand-binding properties and kinase domains, however all share the common extracellular region composed of three immunoglobulin(Ig)-like domains (D1-D3), and thus belong to the immunoglobulin superfamily.The three immunoglobin(Ig)-like domains—D1, D2, and D3—present a stretch of acidic amino acids ("the acid box") between D1 and D2.