RGX-121 Gene Therapy in Children 5 Years of Age and Over With MPS II (Hunter Syndrome)

Inclusion Criteria: Meets any of the following criteria: 1. Has a documented diagnosis of MPS II AND a neurocognitive testing score ≤ 1 ½ standard deviation (SD) from the test normative mean (BSID-III: 77 and MSEL Visual Reception: 35), OR 2. Has a documented diagnosis of MPS II AND has a decline of ≥ 1 standard deviation on serial neurocognitive testing administered between 3 to 36 months apart (BSID-III Cognitive or MSEL Visual Reception), OR 3. Has a relative clinically diagnosed with neuronopathic MPS II who has the same IDS mutation as the participant AND the participant in the opinion of a geneticist has inherited a neuronopathic form of MPS II, OR 4. Has documented mutation(s) in IDS that in the opinion of a geneticist is known to result in a neuronopathic phenotype AND in the opinion of a clinician has a neuronopathic form of MPS II Exclusion Criteria: 1. Has contraindications for intracisternal injection, intracerebroventricular injection, or lumbar puncture 2. Has contraindications for immunosuppressive therapy 3. Has any neurocognitive deficit not attributable to MPS II or diagnosis of a neuropsychiatric condition 4. Has had prior treatment with an AAV-based gene therapy product 5. If receiving ELAPRASE® via intrathecal (IT) administration, must agree to discontinue IT idursulfase for the duration of the study 6. Has experienced a serious hypersensitivity reaction to intravenous (IV) ELAPRASE® 7. Is currently failing to respond to idursulfase (ELAPRASE®) IV due to neutralizing anti-idursulfase antibodies 8. Has received any investigational product within 30 days of Day 1 or 5 half-lives before signing of the ICF, whichever is longer 9. Has a platelet count \<100,000 per microliter (µL), absolute neutrophil count \<1.0 × 103/µL, or aminotransferase (ALT) or aspartate aminotransferase (AST) \>3 × upper limit of normal (ULN) or total bilirubin \>1.5 × ULN at screening unless the participant has a previously known history of Gilbert's syndrome