Oxygen flux, a measure of VATP, is stimulated when Pi is added to the assay buffer; however, no difference was observed between mitochondria isolated from WT and NaPi2a−/− mice, which suggests that intramyocellular Pi regulates VATP and that intramyocellular Pi as a result of hypophosphatemia is likely the cause of reduced VATP in these mice. The gene discussed is SLC34A1; the disease is hypophosphatemia.