['To demonstrate that measurement of endogenous neopterin levels in unstimulated lymphoblasts identifies inherited GTP cyclohydrolase 1 (GCH1) dysfunction and can be a diagnostic test for dopa-responsive dystonia (DRD).', 'DRD results from decreased dopamine biosynthesis due to dysfunctional GCH1. GCH1 is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor for catecholamine synthesis. Mutations in the GCH1 coding region are identified in 60 to 70% of DRD cases; in others, the cause of GCH1 dysfunction is unknown.', 'Using HPLC, we measured endogenous neopterin, the main byproduct of the GCH1 reaction, in lymphoblasts under basal conditions and following GCH1 stimulation conditions.', "In a four-generation family, all identified carriers of dysfunctional GCH1 had basal neopterin levels that were below those of controls. The spouse of one carrier had a mutation in exon 6 of GCH1. Although this man's GCH1 function appeared unaffected by this, his daughter, who was a compound heterozygote with her mother's dysfunctional GCH1 and this mutation, had a phenotype that was more severe than that of typical DRD. Cytokine or phytohemagglutinin (PHA) did not induce GCH1 activity in any carrier of dysfunctional GCH1; controls who did not respond to PHA had increased neopterin levels following cytokine induction."] Does reduced lymphoblast neopterin detect GTP cyclohydrolase dysfunction in dopa-responsive dystonia?