MPS IIIA is caused by the deficiency of the enzyme heparan-N-sulfatase (SGSH, EC 3.10.1.1); MPS IIIB, in turn, is caused by defects in the enzyme N-acetylglucosaminidase (NAGLU, EC 3.2.1.50); in MPS IIIC the protein involved is the transmembrane enzyme, acetyl-CoA:Glucosamine N-acetyltransferase (HGSNAT, EC 2.3.1.78); and, finally, MPS IIID is caused by defects in N-acetyl-glucosamine-6-sulfatase (GNS, EC 3.1.6.14). Here, GNS is linked to mucopolysaccharidosis type 3D.