Mucopolysaccharidosis type IIIB (MPS IIIB) is an genetic disorder that makes the body unable to break down large sugar molecules called glycosaminoglycans (GAGs, formerly called mucopolysaccharides). Specifically, people with this condition are unable to break down a GAG called heparan sulfate. Affected individuals can have severe neurological symptoms, including progressive dementia, aggressive behavior, hyperactivity, seizures, deafness, loss of vision, and an inability to sleep for more than a few hours at a time. MPS IIIB is caused by alterations in the NAGLU gene. This gene provides the instructions for producing an enzyme called N-alpha-acetylglucosaminidase, which is needed to completely break down heparan sulfate. MPS IIIB is inherited in an autosomal recessive manner.