Human and experimental (e.g., BH4-deficient mice, zebrafish gch-/- mutants, and DRD TH knock-in mice) neurochemical data have indicated that dopamine reduction in the striatum of GTPCH-deficient DRD (the most common form of DRD) is caused not only by decreased TH activity owing to low cofactor (BH4) content but also by actual loss of TH protein without degeneration of nigrostriatal dopaminergic neurons. Here, GCH1 is linked to dystonia 5.