For example, the AAV2-aromatic l-amino acid decarboxylase (AADC) enzyme, which catalyzes the conversion of levodopa (primary drug) to dopamine, was delivered via CED to the bilateral putamen to safely increase the AADC levels in Parkinson's disease patients (ClinicalTrials.gov: NCT03065192). This evidence concerns the gene DDC and Parkinson disease.