FGF23 was identified as the last member of FGF23 family. Recent investigations indicate that excess actions of FGF23 cause several hypophosphatemic diseases whereas deficient FGF23 activity results in hyperphosphatemic tumoral calcinosis. These results indicate that FGF23 is a hormone that regulates serum phosphate level in contrast to other FGF family members that work as local factors. Furthermore, FGF23 requires Klotho for its signaling in addition to a canonical FGF receptor. These unique characteristics of FGF23 expanded our knowledge about the diversity of FGF family members and specificity of FGF23.