CASR and hyperparathyroidism: Previous studies failed to find a direct role for CaR in mice homozygous for targeted disruption of exon 5 of the CaR gene, which encodes a segment of the extracellular domain of this receptor.(14,15) These observations may have been confounded by the severe hyperparathyroidism and accompanying hypercalcemia and hypophosphatemia in this animal model.