Catalytic domain of the Protein Tyrosine Kinase, Fibroblast Growth Factor Receptor 3PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Many FGFR3 splice variants have been reported with the IIIb and IIIc isoforms being the predominant forms. FGFR3 IIIc is the isoform expressed in chondrocytes, the cells affected in dwarfism, while IIIb is expressed in epithelial cells. FGFR3 ligands include FGF1, FGF2, FGF4, FGF8, FGF9, and FGF23. It is a negative regulator of long bone growth. In the cochlear duct and in the lens, FGFR3 is involved in differentiation while it appears to have a role in cell proliferation in epithelial cells. Germline mutations in FGFR3 are associated with skeletal disorders including several forms of dwarfism. Some missense mutations are associated with multiple myeloma and carcinomas of the bladder and cervix. Overexpression of FGFR3 is found in thyroid carcinoma. FGFR3 is part of the FGFR subfamily, which are receptor PTKs (RTKs) containing an extracellular ligand-binding region with three immunoglobulin-like domains, a transmembrane segment, and an intracellular catalytic domain. The binding of FGFRs to their ligands, the FGFs, results in receptor dimerization and activation, and intracellular signaling. The binding of FGFs to FGFRs is promiscuous, in that a receptor may be activated by several ligands and a ligand may bind to more that one type of receptor. The FGFR3 subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.