Study J4B-MC-OKAA is a Phase 1/2, multi-center, open-label ascending dose, first-in-human study that will evaluate the safety and effect of intra-cisternal LY3884963 administration on progranulin protein (PGRN) levels in patients with frontotemporal dementia with progranulin mutations (FTD-GRN). Two escalating dose (low dose and medium dose) cohorts are planned, as well as one bridging cohort which will allocate patients to receive either low or medium dose. The duration of the study is 5 years. During the first year, patients will be evaluated for the effect of LY3884963 on safety, tolerability, immunogenicity, biomarkers, and efficacy. Patients will follow up for an additional 4 years to monitor safety and changes on selected biomarkers and clinical outcomes.