The hyperinsulinism-hyperammonaemia syndrome (HHS) was reported in a series of eight patients in 1988. Since then several series of up to 14 patients have been reported. The cause is an activating missense mutation in the GLUD1 gene at chromosome 10q23.3 that encodes glutamate dehydrogenase (GDH). The mutations occur either de novo or with dominant inheritance. The activity of GDH is enhanced by reduction of the inhibitory effect on GDH of guanosine triphosphate (GTP) and …