Loss-of-function mutations of CaSR at 1 or both alleles inhibit CaSR signaling and cause increased PTH secretion, referred to as familial hypocalciuric hypercalcemia 1 (FHH1) and neonatal severe hyperparathyroidism (NSHPT), respectively (26, 27). This evidence concerns the gene CASR and neonatal severe primary hyperparathyroidism.