PTH and primary hyperparathyroidism: While other class C GPCRs, like the CaSR, exhibit Ca2+-sensing properties (Kubo et al., 1998; Wise et al., 1999; Christiansen et al., 2007) it seems unlikely that the parathyroid is equipped with an alternative CaSR since, as described above, mice that are homozygous for either global (Ho et al., 1995) or tissue-selective (Chang et al., 2008) knockouts of the CaSR exhibit a severe, uncompensated form of primary hyperparathyroidism in which the plasma levels of both PTH and calcium are markedly elevated from birth.