Human lysosomal A-N-acetylglucosaminidase is a hydrolase that catalyses the removal of terminal A-N-acetylglucosamine residues from heparan sulfate and heparin. Defects in this gene are the cause of mucopolysaccharidosis type IIIB (MPS-IIIB), also known as Sanfilippo syndrome B. Mucopolysaccharidosis types IIIA, C, and D are caused by mutations in other genes involved in the lysosomal degradation of heparan sulfate. Continuous lysosomal accumulation of heparan sulfate results in the clinical onset of disease, which is typified by severe central nervous system degeneration. Mucopolysaccharidosis type III differs from other mucopolysaccharidoses in that patients usually exhibit mild somatic changes with minimal skeletal abnormalities.