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. Protein Interaction Partner: GCH1, CALML6, CAMKK1, CSNK1G1, CALML5, PACSIN3, RGS19, MAP2K4, MAPK12, MAP2K5, MAPK10, PRKAR2B, PRKAR2A, PPM1B, CSNK2B, CSNK2A1, GIT2, YWHAZ, CNRIP1, RAB26, AHSA1, SP3, RLF, HSPA8, GCHFR, ATP1B1, CDC37, SH3GL2, OSBPL9, RPH3A, Protein Size: 250