Heterozygous loss-of-function mutations lead to one of the three known subtypes of the usually symptomless familial hypocalciuric hypercalcaemia (FHH), while germ-line homozygous inactivating CaSR gene mutations are in the background of neonatal severe hyperparathyroidism (NSHPT). Here, CASR is linked to neonatal severe primary hyperparathyroidism.