After correction with a carbohydrate meal, hypoglycemia was again confirmed the next day at 1 a.m. with an interstitial glucose value of 3.3 mmol/l, a serum glucose value of 2.9 mmol/l, serum insulin 6.3 mE/l, C-peptide 0.44 nmol/l, and proinsulin 13.70 pmol/l, clearly indicating endogenous hyperinsulinism. This evidence concerns the gene INS and hyperinsulinism.