Intraparenchymal delivery of AAV vector–mediated gene therapy has been evaluated in early-phase clinical trials in PD [44, 46-49], Alzheimer disease [3, 50], Canavan disease [51], and aromatic l-amino acid decarboxylase (AADC) deficiency [45, 52], with ongoing trials in lysosomal storage diseases GM2 gangliosidosis [14] and mucopoly saccharidosis type IIIA [22, 53]. The gene discussed is DDC; the disease is Parkinson disease.