PTH and Hypercalcemia: ,28 These mutant mice model the human condition of infantile hypercalcemia type 1, which is caused by homozygous and compound heterozygous inactivating mutations of CYP24A1.29–31 Affected human neonates have hypercalcemia, elevated calcitriol, suppressed PTH, reduced CYP24A1 activity, and reduced 24-hydroxylated metabolites in the circulation.