GCH1 and Parkinson disease: In particular, pathogenic variants in GCH1 (GTP cyclohydrolase I, MIM*600225), the most common cause of levodopa-responsive dystonia (MIM#128230), have been found in patients and families with AD PD characterized by variable disease onset (mean age 43 years), long-term motor complications, and non-motor signs such as cognitive impairment, sleep disorders, hyposmia, and autonomic dysfunction, without dystonia.