FGFR3 (fibroblast growth factor receptor 3), also known as CEK2 and CD333, is a transmembrane receptor tyrosine kinase that plays a role in skeletal development and tumorigenesis. Alternative splicing generates beta isoforms of the receptor which lack the first Ig-like domain as well as IIIb isoforms that have a substitution in the third Ig-like domain. FGFR3 (IIIb) triggers cell proliferation in response to FGF acidic and FGF-9, while FGFR3 (IIIc) shows a wider selectivity that includes FGF acidic, FGF basic/FGF2, FGF-4, -8, -9, -17, -18, -19, and -20. FGF mediated activation of FGFR3 is dependent on the presence of heparan sulfate proteoglycans. FGFR3 functions as a negative regulator of endochondral bone growth, and FGFR3 mutations are associated with chondrodysplasia in humans. The development of many cancers is associated with mutations or dysregulation of FGFR3.