The objective of this study is to evaluate the effect of anti-idursulfase antibodies on idursulfase safety (measured by infusion related adverse events) between patients who develop anti-idursulfase antibodies and patients who do not after long-term idursulfase enzyme replacement therapy (ERT).

This study is being conducted to satisfy post-marketing commitments to monitor anti-idursulfase antibody development in Hunter syndrome patients after long-term idursulfase enzyme replacement therapy. The study will be conducted as a sub-study within the Hunter Outcome Survey (HOS). Hunter syndrome patients in the HOS who have previously received idursulfase as well as treatment-naive patients who will begin idursulfase treatment within 30 days of study enrollment will be included.

Inclusion Criteria:

Patients must meet all of the following criteria to be considered eligible for enrollment:

• The patient is male and enrolled in the HOS (i.e., meets the entry criteria of a documented diagnosis of Hunter syndrome)
• The patient is ≥ 5 years-old
• The patient is on idursulfase treatment or scheduled to begin idursulfase treatment within 30 days of study enrollment
• The patient, patient's parent(s), or patient's legally authorized guardian must have voluntarily signed an Institutional Review Board (IRB)/Independent Ethics Committee (IEC)-approved informed consent form after all relevant aspects of the study have been explained and discussed with the patient, patient's parent(s), or patient's legally authorized guardian.

Exclusion Criteria:

Patients who meet any of the following criteria are not eligible for this study:

• The patient has received biologic/ERT products other than idursulfase, or other investigational product(s) for any reason within 30 days prior to study entry.
• The patient has a life expectancy of < 2 years
• The patient is unable to comply with the protocol, e.g., has a clinically relevant medical condition making implementation of the protocol difficult; has an uncooperative attitude; is unable to return for safety evaluations; or is otherwise unlikely to complete the study, as determined by the Investigator.

Patients with Hunter syndrome