Each of the four variants of the disorder (MPS IIIA, B, C, D) is associated with the deficiency of a specific enzyme involved in HS degradation (N-sulfoglucosamine sulfohydrolase (SGSH), EC 3.10.1.1, MPS IIIA; N-acetyl-α-glucosaminidase (NAGLU), EC 3.2.1.50, MPS IIIB; heparan-α-glucosaminide N-acetyltransferase (HGSNAT), EC 2.3.1.78, MPS IIIC; N-acetylglucosamine-6-sulfatase (GNS), EC 3.1.6.14, MPS IIID). The gene discussed is GNS; the disease is mucopolysaccharidosis type 3C.