Mondo Description A disease characterized by a deficiency in GTP cyclohydrolase I, which leads to a consequent reduction in BH4 and reduces the activity of three BH4-cofactor dependent enzymes - phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. GTP cyclohydrolase I deficiency encompasses a spectrum of disease that includes autosomal dominant and autosomal recessive forms, with severity correlating with the residual enzyme activity. Individuals who are heterozygous for pathogenic variants in GCH1 have symptoms ranging from none (due to reduced penetrance) to dopa-responsive dystonia, which is the most common presentation in symptomatic cases, to rarer neurological presentations such as adult-onset "benign" parkinsonism, various types of focal dystonia, and symptoms simulating cerebral palsy or spastic paraplegia. Hyperphenylalaninemia is absent, and patients respond well to treatment with levodopa. Individuals who are homozygous or compound heterozygous for pathogenic variants in GCH1 typically present with hyperphenylalaninemia, often identified by newborn screening, and severe neurological features and due to very low or undetectable enzyme activity. Treatment with levodopa, BH4, and 5-hydroxytryptophan can improve the symptoms but does not prevent development of severe encephalopathy. Rare cases of GTP cyclohydrolase I deficiency with a phenotype that is intermediate in severity between dopa-responsive dystonia and the severe autosomal recessive form have also been described, supporting the existence a phenotypic spectrum of disease.