The trials include gene therapy trials using adeno-associated virus serotype 2 to deliver aromatic l-amino acid decarboxylase (AAV2-AADC) to treat Parkinon’s disease [13], [14], [15], human acid sphingomyelinase (AAV2-hASM) to treat the lysosomal storage disorder Niemann-Pick Disease [16], [17], glial-derived neurotrophic factor (AAV2-GDNF) to treat Parkinson’s disease [18], [19], a retrovirus to deliver cytosine deaminase for treating brain tumors [20], [21] and liposomal toxins for treating brain tumors [22], [23], [24], [25]. The gene discussed is GDNF; the disease is Parkinson disease.