GCH1 and phenylketonuria: BH4 is sold under the brand names Kuvan and Biopten for supplementation in BH4 deficiency caused by genetic inactivity of GTP-cyclohydrolase-I (GTPCH-I) or 6-pyruvoyltetrahydropterin synthase (PTPS), both enzymes involved in BH4 synthesis as encountered in phenylketonuria [3].