Mucopolysaccharidosis type III (Sanfilippo syndrome) is caused by a defect in one of four genes: N-sulfoglucosamine sulfohydrolase (SGSH) (MPS IIIA OMIM#252900), N-acetyl-alpha-glucosaminidase (NAGLU) (MPS IIIB OMIM#252920), heparan alpha-glucosaminide N-acetyltransferase (HGSNAT) (MPS IIIC OMIM#252930), and N-acetylglucosamine–Sulfohydrolase (GNS) (MPS IIID OMIM#252940). The gene discussed is HGSNAT; the disease is mucopolysaccharidosis type 3A.