Deactivating, typically heterozygous, mutations of the CaSR gene in FHH result in impaired or disabled Ca2+o-dependent inhibition of renal Ca2+ reabsorption, leading to hypocalciuria, and as well as impaired Ca2+o-dependent feedback inhibition of PTH secretion, typically without frank elevations in the serum PTH level as a result of associated increases in Ca2+o (Chu et al., 1995; review: Brown et al., 1995). This evidence concerns the gene PTH and familial hypocalciuric hypercalcemia.