Neonatal severe primary hyperparathyroidism (NSHPT) that presents as severe hypercalcemia, typically occurring in the first 6 months of life, is a rare autosomal recessive disorder that most often is a consequence of the compound heterozygous or homozygous inheritance of two mutationally-inactivated CASR alleles (133). This evidence concerns the gene CASR and hypercalcemia disease.