Mucopolysaccharidosis type II (MPSII, Hunter Syndrome, MIM: 309900) is caused by mutations in the gene encoding the lysosomal enzyme iduronate 2-sulfatase (IDS), with resulting accumulation of the glycosaminoglycans (GAGs), heparan and dermatan sulfate in the lysosomes. The gene discussed is IDS; the disease is mucopolysaccharidosis type 2.