We demonstrate that mutations in the human Ca*+-sensing receptor gene cause familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT), two inherited conditions characterized by altered calcium homeostasis. The Caz+-sensing receptor belongs to the superfamily of seven membrane-spanning G protein-coupled receptors. Three nonconservative missense mutations are reported: two occur in the extracellular N-terminal domain of the receptor; the third occurs in the final intracellular loop. One mutated receptor identified in FHH individuals was expressed in X. laevis oocytes. The expressed wild-type receptor elicted large inward currents in response to perfused polyvalent cations; a markedly attenuated response was obsewed with the mutated protein. We conclude that the mammalian Ca*+-sensing receptor “sets” the extracellular Ca*+ level and is defective in individuals with FHH and NSHPT.