As expected, the endogenous BH4 data were all BLQ (LLOQ 0.5 ng/mL; Table 1) because autosomal recessive 6-PTPS deficiency results in blocking of de novo BH4 biosynthesis (Opladen et al., 2020). The gene discussed is PTS; the disease is hyperinsulinemic hypoglycemia, familial, 4.