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