Gaucher disease type 1 (GD1), caused by mutations in the GBA1 gene, results in β-glucocerebrosidase (GCase) deficiency. Gene therapy is under investigation as a potential treatment option for patients with GD1. The investigational gene therapy FLT201 consists of an adeno-associated virus (AAVS3) encoding a novel GCase variant (GCase-85). Preclinical characterization of FLT201 showed promising results, with GCase-85 being more stable at physiological pH than wild-type GCase and delivered effectively to target tissues. Here, we describe the design of GALILEO-1, a first-in-human Phase I/II safety, tolerability and efficacy study of FLT201 gene therapy in adult patients with GD1. The study results will inform the decision to start a Phase III study of FLT201 in patients with GD1. Clinical Trial Registration: NCT05324943 ( ClinicalTrials.gov )