GCH1 and late-onset Parkinson disease: In contrast to heterozygous pathogenic variants in the guanosine triphosphate cyclohydrolase 1 (GCH1) gene, which cause DYT/PARK-GCH1 (Segawa syndrome), the prototype disorder of Dopa-responsive dystonia, typically presenting as combined dystonia, homozygous variants result in complex dystonia syndromes.