The genetic basis of the majority of FIHP cases remains unexplained, although 15-20% of kindreds are reported to harbour GCM2 mutations, and may have an increased prevalence of multi-gland parathyroid disease, lesser rates of surgical cure, and increased risk of parathyroid carcinoma in such families.13,62–64 The majority of genetically unexplained FIHP kindreds have low numbers of affected individuals,58 suggesting either a low penetrance genetic aetiology, or the chance occurrence of sporadic cases within a family. The gene discussed is GCM2; the disease is familial isolated hyperparathyroidism.