Mucopolysaccharidosis type III (Sanfilippo syndrome, MPS III) comprises a group of clinically indistinguishable, rare, autosomal recessive lysosomal storage diseases caused by the deficiency of one of four enzymes (defining the subtypes A-D) involved in the degradation of heparan sulfate (HS): heparan N-sulfatase (sulfamidase), α-N-acetylglucosaminidase (NAGLU), acetyl-coenzyme A α-glucosaminide-N-acetyltransferase and N-acetylglucosamine-6-sulfatase [1]. The gene discussed is NAGLU; the disease is mucopolysaccharidosis type 3.