The frequency of GCH1 variants was significantly higher in cases with Parkinson’s disease (10/1318; 0.75%) than in individual (UCL-ex controls 1/1635; 0.06%; P = 0.003; OR 12.4 95% CI 1.7–541.1; EVS database 5/4300; 0.11%; P = 0.0004; OR 6.5, 95% CI 2.0–24.5) and combined data sets of controls (6/5935; 0.1%; P = 0.0001; OR 7.5, 95% CI 2.4–25.3). The gene discussed is GCH1; the disease is Parkinson disease.