Even though inherited forms of HPT represent only a small proportion of total cases (< 5%), investigation into the molecular basis of these rare familial syndromes has resulted in considerable insight into the genetics and pathophysiology of both sporadic and familial HPT and spotlighted the importance of genes such as MEN1, CDC73, CASR, GNA11, AP2S1, CDKN1B, and GCM2. It seems very likely that gain- or loss-of-function mutation of other genes, currently unrecognized, can also promote parathyroid tumor formation. The gene discussed is CASR; the disease is tumor of parathyroid gland.