In MPS IIIB, there is a deficiency in the enzyme alpha-N-acetylglucosaminidase (NAGLU) and early clinical symptoms include aggressive behaviour and hyperactivity followed by progressive mental retardation.
Normal alpha-N-acetylglucosaminidase activity was observed in two obligate heterozygotes (the proband's father and her maternal grandmother), suggesting that in addition to the normal and defective alleles, a third, hyperactive allele is also present in this family.