We isolated mouse cDNA encoding a novel FGF (251 amino acids). As this is the 23rd documented FGF, we termed it FGF-23. FGF-23 has a hydrophobic amino terminus (similar to 24 amino acids), which is a typical signal sequence. As expected, recombinant mouse FGF-23 was efficiently secreted by High Five insect cell-infected recombinant baculovirus containing the cDNA, indicating that FGF-23 is a secreted protein. We also isolated human cDNA encoding FGF-23 (251 amino acids), which is highly identical (similar to 72% amino acid identity) to mouse FGF-23. Of human FGF family members, FGF-23 is most similar to FGF-21 and FGF-19 (similar to 24% and similar to 22% amino acid identities, respectively). Human FCF-23 gene was localized on the chromosome 12p13 and found to be tandem linked (within 5.5 kb) to human FGF-6 gene. The expression of FGF-23 mRNA in mouse adult tissues was examined by real-time quantitative polymerase chain reaction. FGF-23 mRNA was mainly expressed in the brain and thymus at low levels. The localization of FGF-23 mRNA in the brain was examined by in situ hybridization, FGF-23 mRNA in the brain was found to be preferentially expressed in the ventrolateral thalamic nucleus. Therefore, FGF-23 is expected a unique FGF that plays roles in the function of the ventrolateral thalamic nucleus.