QDPR and hyperinsulinemic hypoglycemia, familial, 4: Although the BH4 amount did not differ between DA neuronal cultures using DHPR deficiency iPSCs and DHPR deficiency-corrected iPSCs, since it was reported that BH2 can act as an inhibitor of the aromatic amino acid hydroxylase (22), accumulated BH2 in DA neuronal culture using DHPR deficiency iPSCs may lead reduced the DA level via inhibition of TH (Supplementary Material, Fig. S3F).