In this paper, our purpose was to highlight how important biochemical studies on the functional and structural features of AADC have been for the identification of the molecular defects of the variants associated with AADC deficiency either in homozygosis or in heterozygosis, as well as, as a consequence, to provide an experimental framework useful to suggest appropriate therapeutic treatments. This evidence concerns the gene DDC and hyperinsulinemic hypoglycemia, familial, 4.