GCH1 and Parkinson disease: Many Mendelian forms of PD adopt the PARK gene nomenclature [e.g., SNCA (PARK1), PRKN (PARK2), ATP13A2 (PARK9)], but these labels have not always been applied systematically; therefore, some genes causing familial parkinsonian syndromes were initially classified using other prefixes, including for dystonias [GCH1 (DYT5a)], ataxias [ATXN2 (SCA2)], or neurodegeneration with brain iron accumulation [PLA2G6 (NBIA2)].