IDS and lysosomal storage disease: Mucopolysaccharidosis type II (MPS II; Hunter syndrome; OMIM 309900) is an X-linked recessive lysosomal storage disorder caused by a deficiency of iduronate-2-sulfatase (IDS) activity, which is involved in the lysosomal degradation of the glycosaminoglycans (GAGs), dermatan sulfate (DS) and heparan sulfate (HS).