Extracellular calcium-sensing receptor is a G-protein coupled receptor which senses extracellular levels of calcium ion. In the parathyroid gland, the calcium-sensing receptor controls calcium homeostasis by regulating the release of parathyroid hormone (PTH).

Mutations that inactivate CASR cause familial hypocalciuric hypercalcemia (FHH) (also known as Familial Benign Hypercalcemia because it is generally asymptomatic and does not require treatment), whereas mutations that activate CASR are the cause of autosomal dominant hypocalcemia. An alternatively spliced transcript variant encoding 1088 aa has been found for this gene, but its full-length nature has not been defined.