GCK and neonatal diabetes mellitus: Heterozygous inactivating (loss of function) mutations in the GCK locus result in MODY2 or GK-MODY, homozygous inactivating mutations produce permanent neonatal diabetes mellitus, and heterozygous activating (gain of function) GCK mutations cause autosomal dominant hyperinsulinism (persistent hyperinsulinaemic hypoglycaemia of infancy).