Our in vivo studies using these 2 mouse models demonstrated that typical clinical manifestations of XLHR occurred in a gene–dosage and Phex activity-dependent manner, including growth retardation, skeletal dysplasia (rickets/osteomalacia), and hypophosphatemia, which was accompanied by elevated serum FGF23 and ALP levels. The gene discussed is FGF23; the disease is hypophosphatemia.