This study will primarily investigate the safety of increasing AADC levels in the striatum via AADC gene delivery. The hAADC gene is packaged into a gene transfer vector derived from a common, non-pathogenic virus (AAV2) to which >90% of humans have been exposed. This investigational drug, termed VY-AADC01, will be injected directly into the striatum during a neurosurgical procedure that is performed with real-time MRI imaging to monitor delivery.