Heterozygous mutations in the IDS gene, accompanied by an extreme skewing of X-inactivation, leading to complete or nearly complete preferential inactivation of the X chromosome carrying the normal IDS allele, leaving the variant IDS allele active in all cells of a patient, have been detected in most of the female MPS II cases described in the literature (Table S2) [140,141,142,143,144,145,146,147,148,149]. This evidence concerns the gene IDS and mucopolysaccharidosis type 2.