A Study of Potential Treatment-Responsive Biomarkers and Clinical Outcomes in Hunter Syndrome

Key Inclusion Criteria (Part 1): * Participants aged 2 through 10 years * nMPS II subgroup: participants with a development quotient (DQ) \<85 and/or a decline of at least 7.5 points in DQ, assessed at least 6 months apart, or with the same genetic mutation as a blood relative with confirmed nMPS II Key Inclusion Criteria (Part 2): * Participants aged 2 through 30 years * nMPS II subgroup: patients with an age-adjusted DQ \<85 and/or a decline of 10 points or more in DQ in the previous 6 months or more, or with the same genetic mutation as a blood relative with confirmed nMPS II * Scheduled to undergo general anesthesia or CSF sampling for non-study-related medical reasons and parent(s)/legally authorized representative consent to donate CSF for research purposes during that procedure, or an adult patient is able to provide consent and agrees to participation in the study for CSF collection/donation Key Inclusion Criteria (Part 3): * nMPS II participants aged \<8 years Key Inclusion Criteria (Part 4): * nnMPS II participants aged 6 to 17 years Key Inclusion Criteria (Part 5): * Participants aged ≤ 3 years * Have undetermined MPS II phenotype * Do not have a large deletion(s) or rearrangement(s) in the IDS gene or other definitive mutation indicative of nMPS II * Do not have a DQ \< 85 at the screening/baseline neurocognitive assessment and/or a documented decline of at least 7.5 points in DQ in the previous 6 to 18 months * Do not have the same IDS gene variant as a blood relative with confirmed nMPS II or nnMPS II Key Inclusion Criteria (Part 6): * nMPS II participants aged 1 to 17 years * Have received an MPS II gene therapy or allogeneic HSCT \> 12 months prior to screening * Have a post-HSCT or post-gene therapy DQ \< 85 at the screening/baseline neurocognitive assessment and/or a documented decline of at least 7.5 points in DQ in the previous 6 to 18 months Key Exclusion Criteria (All Parts): * Have unstable medical condition that would make participation in the study unsafe or would interfere with necessary medical care * Have received any central nervous system (CNS)-targeted MPS II investigational therapy within the previous 6 months