There are 3 genetic types of FHH based on chromosome location. FHH type 1 accounts for 65% of cases and is due to inactivating mutations in the CASR gene, localized to 3q21.1. This gene encodes the calcium-sensing receptor (CaSR). Loss of CaSR function results in a reduction in the sensitivity of parathyroid and renal cells to calcium levels so hypercalcemia is perceived as normal. The other 35% have either a mutation GNA11 (19p13.3) seen in FHH type 2 or AP2S1 (19q13.2-q13.3) seen in FHH type 3 (see these terms) or in genes not yet discovered. FHH is rarely caused by auto-antibodies against CaSR in those without a mutation.