Since GAMT deficiency (GAMT-D) is considered to be the most severe CDS, we summarize data regarding biochemical and behavioral phenotypes of GAMT knockout (KO) mice in order to highlight similarities and possible differences between the mouse model and human pathology, and the behavioral alterations comparable with clinical symptoms reported in GAMT-D patients. The gene discussed is GAMT; the disease is hyperinsulinemic hypoglycemia, familial, 4.