Fibroblast growth factor 23 (FGF-23), which is produced by bone cells, is a novel member of a large family of related proteins. Its gene encodes a 251 amino acid protein. The amino-terminal portion of FGF-23 (aa 1-24) is hydrophobic and is likely to serve as a signal peptide allowing its secretion into the blood circulation. Its carboxyl-terminal portion (aa 180-251) shares only limited amino acid homology with other members of the FGF family of proteins.