FGF R3 (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. FGF R3 (IIIb) triggers cell proliferation in response to FGF acidic and FGF-9, while FGF R3 (IIIc) shows a wider selectivity that includes FGF acidic, FGF basic, FGF-4, -8, -9, -17, -18, -19, and -20. FGF mediated activation of FGF R3 is dependent on the presence of heparan sulfate proteoglycans. FGF R3 functions as a negative regulator of endochondral bone growth, and FGF R3 mutations are associated with chondrodysplasia in humans. The development of many cancers is associated with mutations or dysregulation of FGF R3.