CAMPSIITE™ RGX-121 Gene Therapy in Subjects With MPS II (Hunter Syndrome)

Part 1 Inclusion Criteria: * The subject's legal guardian(s) is (are) willing and able to provide written, signed informed consent after the nature of the study has been explained, and prior to any research-related procedures * Is a male ≥4 months to \< 5 years of age on Day 1 * Must meet any of the following criteria: * Has a documented diagnosis of MPS II and a has a neurocognitive testing score ≤ 77 (Bayley or Kaufman), OR * 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 (Bayley or Kaufman) OR * Has a relative clinically diagnosed with severe MPS II who has the same IDS mutation as the subject AND in the opinion of a geneticist has inherited a severe form of MPS II OR * Has documented mutation (s) in IDS that in the opinion of a geneticist is always known to result in a neuronopathic phenotype AND in the opinion of a clinician has a severe form of MPS II Part 2 Inclusion Criteria: * The subject's legal guardian(s) is (are) willing and able to provide written, signed informed consent after the nature of the study has been explained, and prior to any research-related procedures * Is a male ≥4 months to \< 5 years of age on Day 1 * Has a documented diagnosis of neuronopathic MPS II. Neuronopathic MPS II can be documented with any of the following methods: * Has a BSID-III Cognitive Composite score at or below -1 SD (85) from normative mean * Has two co…