FGF23 and Hypocalcemia: Therefore, Fgf23−/−/VDRΔ/Δ and Kl−/−/VDRΔ/Δ double mutant mice can be used to examine the roles of FGF23 and Klotho at older ages by keeping these mice on a rescue diet rich in calcium, phosphorus, and lactose (150, 151) with the goal of preventing hypocalcemia and severe hyperparathyroidism due to the non-functioning VDR status.