A number sign is used with this entry because the phenotype is caused by mutation in the gene encoding N-sulfoglucosamine sulfohydrolase (SGSH). The Sanfilippo syndrome, or mucopolysaccharidosis III, is a lysosomal storage disease due to impaired degradation of heparan sulfate.

The Sanfilippo syndrome is characterized by severe central nervous system degeneration, but only mild somatic disease. Onset of clinical features usually occurs between 2 and 6 years; severe neurologic degeneration occurs in most patients between 6 and 10 years of age, and death occurs typically during the second or third decade of life. Type A has been reported to be the most severe, with earlier onset and rapid progression of symptoms and shorter survival