Abstract: The present invention concerns fibroblast growth factor 9 (FGF9) as a high affinity ligand for fibroblast growth factor receptor 3 (FGFR3) which ligand is capable of binding and activating FGFR3 in a specific manner. The present invention is also directed to methods for detection of FGFR3 by utilizing FGF9, as well as to pharmaceutical compositions for modulating the activity of FGFR3 comprising as an active ingredient FGF9, antagonists thereof or FGF binding agents which are capable of neutralizing native circulating FGF9. The present invention further concerns novel recombinant mouse and chicken FGF9, expression vectors comprising these recombinant FGF9s and a transgenic animal transformed with said expression vectors.