Glucosamine (N-acetyl)-6-sulfatase (GNS), also known as G6S, a hydrolase, which is one of the enzymes involved in heparan sulfate catabolism leading to lysosomal storage. GNS is required for the catabolism of the glycosaminoglycans (GAG) including heparin, heparan sulphate, and keratan sulphate through the hydrolysis of 6-sulfate group from the N-acetyl-D-glucosamine 6-sulfate units. Mucopolysaccharidosis type IIID (MPS IIID) is the least common of the four subtypes of Sanfilippo syndrome. It is caused by a deficiency of N-acetylglucosamine-6-sulphatase. A mutation in GNS resulting in MPS IIID indicates the potential utility of molecular diagnosis for this rare condition. As the least common type of the four subtypes of Sanfilippo syndrome, MPS IIID has profound mental deterioration, hyperactivity, and relatively mild somatic manifestations.