Neurochemical findings in an asymptomatic GCH1 mutation carrier versus symptomatic cases suggest that there may be additional genetic and/or environmental factors modulating the regulatory BH4 effect on TH stability and that the extent of striatal protein loss in TH (rather than that in GTPCH) may be critical in determining the symptomatic state of GTPCH-deficient DRD. The gene discussed is GCH1; the disease is dystonia 5.