RGX-121-3102 Gene Therapy in Participants With MPS II (Hunter Syndrome)

Inclusion Criteria: * The participant'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 study-related procedures being performed. * Is a male ≥ 4 months to \< 5 years of age on Day 1. * Has a documented diagnosis of MPS II, with a confirmed neuronopathic phenotype. * Has a BSID-III Cognitive Composite score at or below -1SD (85) from the normative mean. * Has 2 consecutive neurodevelopment assessments that support a decline on MSEL Visual Reception, Expressive Language, or Fine Motor, or BSID-III Cognitive, Expressive Communication, or Fine Motor of ≥ 1 SD on serial neurodevelopment testing administered between 3 to 36 months apart. Evidence for neurodevelopmental decline can be provided from historical medical records. * Has a relative clinically diagnosed with neuronopathic MPS II who has the same IDS variant(s) as the participant AND the participant, in the opinion of a geneticist, has inherited a neuronopathic form of MPS II. Evidence for support of a relative with neuronopathic MPS II should be provided through medical record documentation of neurodevelopmental function at or below -2 SD from the normative mean. If standard scores are unavailable to document SD from the normative mean, a developmental quotient (AEq/chronological age × 100) of ≤ 60 can be used to document neuronopathic MPS II. * Has documented variant(s) in IDS known to result in a neuronopathic phe…