The reported phenotypic overlap between PD patients with PARKIN mutation and rare forms of dystonia-parkinsonism, such as levodopa-responsive dystonia (DRD), prompts screening for PARKIN mutations along with GTP cyclohydrolase 1 and tyrosine hydroxylase in clinically diagnosed DRD patients (Tassin et al., 2000; Wu et al., 2005, 2008; Schneider and Bhatia, 2010; Potulska-Chromik et al., 2017). Here, GCH1 is linked to Parkinson disease.