MPS II is a rare X-linked recessive genetic disease caused by mutations in the iduronate-2-sulfatase gene (IDS). Enzyme replacement therapy (ERT) with recombinant idursulfase (ELAPRASE®) is the only approved product for the treatment of Hunter syndrome; however, ERT as currently administered does not cross the blood brain barrier and is therefore unable to address the unmet need in MPS II patients with CNS (neurocognition and behavior) involvement. This is an observational study to document prospectively disease manifestation and neurocognitive course in pediatric patients with a clinical presentation consistent with neuronopathic ("severe") MPS II undergoing current standard of care for their condition. Approximately forty pediatric subjects who have severe MPS II will be enrolled. Changes in neurodevelopmental parameters of cognitive, behavioral, and adaptive function over time will be the primary focus for a duration of 104 weeks.
