The four types of MPS III are MPS IIIA (OMIM: 252900; N-sulfoglucosamine sulfohydrolase, SGSH, deficiency); MPS IIIB (OMIM: 252920; α-N-acetyl glucosaminidase, NAGLU, deficiency); MPS IIIC (OMIM: 252930; heparan sulfate acetyl CoA, α-glucosaminide N-acetyltransferase, HGSNAT, deficiency); and MPS IIID (OMIM: 252940, α-N-acetylglucosamine-6-sulfatase, GNS, deficiency, the rarest MPS III type) [2,6]. The gene discussed is GNS; the disease is mucopolysaccharidosis type 3D.