The purpose of this study is to look at the safety and tolerability of RBX-121, a one-time gene therapy, in subjects with MPS II (Hunter Syndrome). You may be eligible for this study if you are a male between the ages of 4 months to 5 years old. Reimbursement for travel and research procedures will be provided. This study involves blood draws, interviews, urine collection, genetic testing, general anesthesia, lumbar punctures, MRI, gene therapy, lumbar puncture, ultrasound, ECG/echocardiogram and physical and neurological examinations. If you have questions or would like to learn more about the study, please contact the study team at MetabolismResearch@email.chop.edu.