CASR and Hypercalcemia: Since the release of parathyroid hormone (PTH) from parathyroid chief cells is tightly regulated by the calcium-sensing receptor (CASR), a cell surface-expressed G protein-coupled receptor (GPCR) belonging to GPCR family C (2), mutation in the germline of CASR or other genes transducing and propagating the CASR signal can also result in heritable syndromes associated with hypercalcemia and PTH levels that are high or inappropriately normal.