The second part of the study (Phase 2) will provide more information about the safety of CERE-120 and also evaluate if it is beneficial in the treatment of Parkinson's disease. In Phase 2, approximately 52 people with Parkinson's disease will be enrolled. Half of the subjects will receive CERE-120 and the other half will undergo a "placebo" surgery where no medication will be injected. Treatment assignment will be random (like the flip of a coin). Participants in both phases of the study will be followed for three years after surgery.