SMPD1 and dopa-responsive dystonia: Some examples are variants in the glucocerebrosidase (GBA) [40], the sphingomyelin phosphodiesterase 1 (SMPD1) [41], and the GTP cyclohydrolase 1 (GCH1) [42] genes, responsible for Gaucher's disease, Niemann-Pick A disease, and dopa-responsive dystonia (DRD), respectively.