Several gene mutations have been shown to cause various forms of familial isolated hypoparathyroidism, and while mutations in preproPTH and CASR genes affect PTH secretion, patients with homozygous inactivating GCM2mutations or heterozygous dominant-negative GCM2 mutations develop hypoparathyroidism, presumably from parathyroid dysgenesis (15, 16, 19). The gene discussed is PTH; the disease is hypoparathyroidism.