Analysis of entheses in mice null for both Cyp27b1 and Fgf23 (C–/–F–/–), which have normal serum calcium and phosphate levels when fed a rescue diet, will differentiate whether impaired 1,25D production due to high FGF23 levels or actions specific to FGF23 lead to XLH enthesopathy. The gene discussed is FGF23; the disease is X-linked hypophosphatemia.