In conclusion, loss of Cyp24a1 in fetal mice causes hypercalcemia, modest hypophosphatemia, and increased calcitriol and FGF23, but no alteration in skeletal development, and it predicts that human fetuses bearing homozygous or compound heterozygous inactivating mutations of CYP24A1 will also be hypercalcemic in utero but with normal skeletal development. Here, FGF23 is linked to hypercalcemia disease.