FGF23 and hyperphosphatemia: To test the hypothesis that increased NaPi2a activity is responsible for the severe hyperphosphatemia in Fgf-23−/− animals, we generated a new mouse model deficient in both the Fgf-23 and the NaPi2a genes (Fgf-23−/−/NaPi2a−/− compound mutants) by interbreeding heterozygous- Fgf-23 and NaPi2a mice.