GLUD1 and hyperinsulinism: Hyperinsulinism hyperammonemia syndrome is caused by a heterozygous missense amino acid substitution on the GLUD1 gene that results in an increase in glutamate dehydrogenase activity by impairing allosteric inhibition by guanosine triphosphate (GTP).1 GDH activity occurs exclusively in the pancreatic islets, liver, kidney, and brain.2 Increased GDH activity is responsible for increased insulin secretion as well as increased renal ammoniagenesis.3 Approximately 70% of the HI/HA mutation occur de novo, whereas the other 30% occurs in an autosomal dominant fashion.