It is classified as MPS type III, though it is caused by four different genetic defects, determining subtypes A, B, C and D. In each subtype of MPS III, the primary storage GAG is heparan sulfate (HS), but mutations leading to A, B, C, and D subtypes are located in genes coding for heparan N-sulfatase (the SGSH gene), α-N-acetylglucosaminidase (the NAGLU gene), acetyl-CoA:α-glucosaminide acetyltransferase (the HGSNAT gene), and N-acetylglucosamine-6-sulfatase (the GNS gene), respectively.