Spontaneous hyperinsulinemic hypoglycemia in adults is most frequently caused by sporadic, solitary pancreatic beta-cell tumors, whereas hyperinsulinemic hypoglycemia in childhood is commonly caused by generalized beta-cell dysfunction.¹ Mutations in the beta-cell sulfonylurea-receptor (SUR1) gene or inward-rectifying potassium-channel (Kir6.2) gene were found in some patients.²–⁷ A distinct syndrome of hyperinsulinism with hyperammonemia was recently described,⁸,⁹ apparently caused by mutations in the glutamate dehydrogenase gene.¹⁰ However, many sporadic and familial cases of hyperinsulinism remain unexplained. Some may be due to somatic mutations in other genes, as suggested by reports of autosomal dominant familial hyperinsulinism that was . . .