CERE-120 has been previously studied in 50 people with Parkinson's enrolled in two clinical studies. These patients have been followed for over 2 years (some for almost 5 years) and so far CERE-120 has been well tolerated. Whereas CERE-120 was not better than placebo on the primary efficacy measure in a completed study, several measures of improvement suggested a modest benefit. In order to build upon these findings and increase the chances for a stronger clinical benefit, a dose increase and modifications in the delivery targets have been implemented in the current study.