CASR and familial isolated hyperparathyroidism: Heterozygous and homozygous inactivating CASR mutations are occasionally reported in kindreds with apparent FIHP (i.e., with clinical, biochemical and histological features more typical of PHPT than FHH), in whom parathyroid surgery has improved hypercalcaemia.59–61 However, the finding of an inactivating CASR mutation in a kindred with hereditary hypercalcaemia usually indicates a diagnosis of FHH (see below).