Heterozygous inactivating (loss of function; LoF) alleles in GCK cause elevated fasting plasma glucose levels or GCK-MODY, while activating (gain of function; GoF) alleles cause the opposite phenotype of persistent hyper-insulinemic hypoglycemia of infancy (PHHI) [7,9]. The gene discussed is GCK; the disease is congenital isolated hyperinsulinism.