This gene encodes a member of the GTP cyclohydrolase family. The encoded protein is the first and rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis, catalyzing the conversion of GTP into 7,8-dihydroneopterin triphosphate. BH4 is an essential cofactor required by aromatic amino acid hydroxylases as well as nitric oxide synthases. Mutations in this gene are associated with malignant hyperphenylalaninemia and dopa-responsive dystonia. Several alternatively spliced transcript variants encoding different isoforms have been described, however, not all variants give rise to a functional enzyme.,GCH1,DYT14,DYT5,DYT5a,GCH,GTP-CH-1,GTPCH1,HPABH4B,Cancer,Signal Transduction,Cell Biology & Developmental Biology,Endocrine & Metabolism,Amino acid metabolism,Neuroscience,GCH1