QDPR and hyperinsulinemic hypoglycemia, familial, 4: Although DHPR activity in DHPR deficiency iPSCs and DHPR deficiency-corrected iPSCs was undetectable, an increase of oxidized biopterins in DA neuronal culture using DHPR deficiency iPSCs suggests that DHPR activity in DHPR deficiency cells was lower than that in DHPR deficiency-corrected cells, because low DHPR activity causes inefficient reduction of quinonoid dihydrobiopterin (qBH2) to BH4, resulting in increases of BH2 and biopterin.