A supervised fast was performed and terminated after 3 h with symptoms of hypoglycemia, a glucose level of 2.7 mmol/L, an insulin level of 10.7 mU/L (reference range 2.6–24.9 mU/L), and a C-peptide of 0.32 nmol/L (reference range 0.27–1.27), confirming endogenous hyperinsulinemic hypoglycemia. The gene discussed is INS; the disease is Hypoglycemia.