CASR is a key molecule in parathyroid pathophysiology: It mediates the parathyroid cell sensitivity to extracellular calcium concentrations resulting in inhibition of the PTH release; it further acts as an oncosuppressor in parathyroid cells as CASR inactivating mutations induce parathyroid hyperplasia in the set of familial hypocalciuric hypercalcemia type 1 (FHH1; OMIM#145980). The gene discussed is CASR; the disease is familial hypocalciuric hypercalcemia 1.