IDS and mucopolysaccharidosis type 2: This, together with the suggestion that low residual activity may be sufficient to relieve central nervous system symptoms of MPS II [14], may suggest a plausible mechanism for the treatment response observed in patients younger than 6 years at baseline with missense IDS variants, namely that administration of idursulfase-IT may supplement the residual enzyme activity enough to reduce lysosomal accumulation of GAGs.