The purpose of this study is to evaluate the safety and potential benefits of CERE-120 in the treatment of Parkinson's disease. CERE-120 is an experimental drug that consists of an adeno-associated virus (AAV) that was engineered to carry the human gene for neurturin, a neurotrophic (growth) factor. Similar to other growth factors (such as GDNF), neurturin is capable of restoring function and protecting brain cells from further damage. The virus used in CERE-120 is not known to cause disease in people.