GCM2 and familial isolated hyperparathyroidism: For example, loss-of-function Glial Cells Missing Transcription Factor 2 (GCM2) variants are associated with isolated HP, whilst activating GCM2 variants are reported in familial isolated hyperparathyroidism (FIHP).12,13 Similarly, whilst inactivating CASR and GNA11 mutations are associated with FHH types 1 and 2, gain of function variants in these genes result in autosomal dominant hypocalcaemia (ADH) types 1 and 2, respectively.14,15 Finally, the severity of phenotype may be determined by whether variants are inherited in a dominant or recessive manner.