GNS and isovaleric acidemia: There are eleven MPSs-related enzymes, including α-l-iduronidase (IDUA) for MPS I, iduronate sulfatase (IDS) for MPS II, heparan N-sulfatase (SGSH) for MPS IIIA, α-N-acetylglucosaminidase (NAGLU) for MPS IIIB, heparan acetyl CoA: α-glucosaminide N-acetyltransferase (HGSNAT) for MPS IIIC, N-acetylglucosamine-6-sulfatase (GNS) for MPS IIID, N-acetylgalactosamine-6-sulfatase (GALNS) for MPS IVA, β-galactosidase (GLB1) for MPS IVB, N-acetylgalactosamine-4-sulfatase (ARSB) for MPS VI, β-glucuronidase (GUSB) for MPS VII and hyaluronidase (HYAL1) for MPS IX (2) (Fig. 1).