Rarely, mutations in the genes which encode glucokinase (GCK), glutamate dehydrogenase (GLUD1), 3-hydroxyacyl-coenzyme A dehydrogenase (HADH), hepatocyte nuclear factor 4 (HNF4A), hepatocyte nuclear factor 1 (HNF1A), monocarboxylate transporter 1 (SLC16A1), and the mitochondrial inner membrane protein UCP2 (UCP2) have been reported to cause hyperinsulinemic hypoglycemia (1). Here, GCK is linked to hyperinsulinemic hypoglycemia.