About 70% of the patients have the severe type of MPS II, presenting CNS-related symptoms [93] and a mutation of recombination or large deletion of the IDS gene because of the pseudogene IDS2 located on the telomeric side of chromosome X. The remaining 30% suffer from the attenuated type of MPS II, which does not affect the CNS. This evidence concerns the gene IDSP1 and mucopolysaccharidosis type 2.