Study IDS116406 will be a non-interventional, phlebotomy study in Hunter Syndrome patients who are currently being treated with idursulfase, an enzyme replacement therapy, and in at least a single patient who is naïve to treatment, if possible to recruit. All patients enrolled into the study will have a single blood draw for the analysis of antibodies induced by this enzyme replacement therapy (idursulfase). Patient samples with positive responses to antibodies induced by idursulfase will be used to further evaluate whether the antibodies induced by idursulfase bind to GSK2788723 molecules in vitro and if these antibodies neutralize the bioactivity of GSK2788723 in vitro. Each subject will have a screening visit, which may occur at their regularly scheduled out-patient visit. If the patient consents to participate in the study, a blood sample (total volume of approximately 3mL) for immunogenicity analysis will be drawn before their current treatment infusion

Study IDS116406 will be a non-interventional, phlebotomy study in Hunter Syndrome patients who are currently being treated with idursulfase, an enzyme replacement therapy, and in at least a single patient who is naïve to treatment, if possible to recruit. All patients enrolled into the study will have a single blood draw for the analysis of antibodies induced by this enzyme replacement therapy (idursulfase). Patient samples with positive responses to antibodies induced by idursulfase will be used to further evaluate whether the antibodies induced by idursulfase bind to GSK2788723 molecules in vitro and if these antibodies neutralize the bioactivity of GSK2788723 in vitro. Each subject will have a screening visit, which may occur at their regularly scheduled out-patient visit. If the patient consents to participate in the study, a blood sample (total volume of approximately 3mL) for immunogenicity analysis will be drawn before their current treatment infusion

Inclusion Criteria:

A subject will be eligible for inclusion in this study only if patient meets criteria 1 and 2 or 1 and 3:

• Diagnosed with Hunter syndrome
• Patients with Hunter syndrome who are being treated with idursulfase
• Patients with Hunter syndrome who are naïve of idursulfase treatment (if possible*) *This study will also attempt to recruit 1 patient who has never received idursulfase. If a naïve patient has not been identified within an agreed amount of time between GSK and the investigator, the study will concluded without this sample

Exclusion Criteria:

• Subjects who are currently participating in another clinical trial are not permitted to be enrolled in this study.

Approximately 11 subjects will be enrolled in the study.