The enzyme deficiencies underlying all four subtypes of the disease, MPS III A, B, C and D, were subsequently identified: MPS IIIA was found to be caused by defects in N-sulfoglucosamine sulfohydrolase (SGSH) (EC 3.10.1.1) [5], MPS IIIB, by defects in N-acetyl-α-D-glucosaminidase (NAGLU) (EC 3.2.1.50) [6], MPS IIIC, by defects in acetyl-CoA:alpha-glucosaminide N-acetyltransferase (HGSNAT) (EC 2.3.1.78) [7], MPS IIID, by defects in N-acetylglucosamine-6-sulfate sulfatase (GNS) (EC 3.1.6.14) [8]. Here, SGSH is linked to mucopolysaccharidosis type 3A.