Inclusion Criteria: * Males and females, ages 35 to 70 years old (inclusive) * A diagnosis of idiopathic Parkinson's disease based on UK Brain Bank criteria, including bradykinesia and at least 1 of the following PD features: resting tremor or rigidity * A Hoehn and Yahr score of no greater than 3 in the "off" condition at Screening * A robust response to dopaminergic therapy as judged by the investigator based on the UPDRS Part III: Motor Examination * Experiencing motor complications despite adequate antiparkinsonian therapy * A stable, optimized regimen of antiparkinsonian medications and stable parkinsonian features for at least 6 weeks prior to Screening * Subject is willing not to undergo DBS for at least 12 months after the study surgical procedure (Phase 1 subjects) or while the study is blinded (Phase 2 subjects) and the investigator believes that this is medically acceptable * Medically fit to undergo the study surgical procedure as determined by medical history, clinical and laboratory evaluations, and any other pre-surgical evaluations that are standard at the institution where the subject will undergo surgery * Physically and mentally capable of performing all protocol-specified assessments and complying with the study visit schedule * Subjects must be able to travel to study visits alone or able to identify a partner or caregiver who agrees to accompany the subject to the study visits * Females of childbearing potential must have a negative β-HCG pregnancy test at Screening and again before surgery on Day 0 * All subjects, both male and female, must agree to practice adequate barrier method contraception for at least 6 months after the surgical procedure * Provides written informed consent to participate before any study-specific procedures are conducted Exclusion Criteria: * Atypical or secondary parkinsonism, including, but not limited to, multiple system atrophy (MSA) or progressive supranuclear palsy * Any subject for whom participation in the study would pose a substantial safety risk * Any condition that would compromise the ability of the subject to undergo study procedures, including allergy to gadolinium * Presence of any known brain abnormality that could interfere with the assessment of safety or efficacy or represents a surgical risk to the subject * Evidence of significant brain atrophy on the Baseline MRI * History of any cancer other than basal or squamous cell skin cancer within the 3 years prior to Screening * Any chemotherapy, cytotoxic therapy, or immunotherapy (e.g., IL-2, IL-12, interferon) within the 3 months prior to Screening * Any prior treatment for PD with a procedure involving intracranial surgery or implantation of a device (e.g. DBS, pallidotomy) * Any prior treatment for a neurological or psychiatric disorders with a procedure involving the implantation of a device (e.g. spinal cord stimulator, vagus nerve stimulator) * History of any prior gene transfer therapy * Treatment with any investigational agent within the 3 months prior to Screening * Anticipated need for antiplatelet agents or anticoagulation therapy, including gingko biloba, during the 10 days prior to the projected surgery date * Any vaccinations within the 30 days prior to the projected surgery date Note: Vaccinations are not allowed for 30 days after the surgical procedure, unless deemed necessary by the investigator for the subject's well-being * Not likely to be available for the duration of the trial, likely to be noncompliant with the protocol, or who are deemed unsuitable by the investigator for any other reason * Participation in a previous surgical treatment study for Parkinson's disease