An Observational Study Evaluating Anti-Idursulfase Serum Antibody Response in Hunter Syndrome Patients

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.