GCH1 and Parkinson disease: From the genetics perspective, parkinsonism is commonly seen in carriers of mutations in DYT genes, such as TAF1 (DYT3), ATP1A3 (DYT12) and PRKRA (DYT16), but especially in those involved in the dopamine synthesis pathway, GCH1 (DYT5a) and TH (DYT5b) [11].