After 2–8 years of treatment with IV idursulfase in studies in patients with neuronopathic MPS II, improvements were commonly observed in uGAG levels and hepatomegaly [29,33,36,38,40]; other outcome improvements (e.g., for growth, splenomegaly, or cardiac disease) were reported but require further evaluation [29,33,37,42,47]. The gene discussed is IDS; the disease is mucopolysaccharidosis type 2.