,73,74 For instance, genetic mutations in an allele of the GCH1 or mutations in both alleles of sepiapterin reductase (enzymes involved in BH4 synthesis) cause dopa-responsive dystonia due to dopamine deficiency in the brain, and the pharmacological treatment is depicted by the administration of levodopa/carbidopa and 5-hydroxytryptophan plus BH4.75–77. Here, GCH1 is linked to dopa-responsive dystonia.