Study J3Z-MC-OJAA is a Phase 1/2a, multicenter, open-label, ascending dose, first in-human study that will evaluate the safety of intracisternal LY3884961 administration in patients with moderate to severe Parkinson's disease with at least 1 pathogenic GBA1 mutation. Two dose level cohorts of LY3884961 are planned (Dose Level 1 and Dose Level 2). The duration of the study is 5 years. During the first year, patients will be evaluated for the effect of LY3884961 on safety, tolerability, immunogenicity, biomarkers, and clinical efficacy measures. Patients will continue to be followed for an additional 4 years to continue to monitor safety as well as selected biomarker and efficacy measures.