Mucopolysaccharidosis III (MPSIII) or Sanfilippo syndrome is an autosomal recessive disorder of lysosomal metabolism. MPS III is caused by mutations in genes that encode for the enzymes involved in the degradation of heparan sulfate. It is classified into 4 subtypes (MPSIII A-D). MPS IIIB is induced by mutations in the gene encoding the alpha-N-acetylglucosaminidase enzyme. We report a 6-year-old boy with phenotypic findings of Sanfilippo syndrome type B, such as mild coarse facie, clear corneas, hirsutism, hepatomegaly, mild joint stiffness and mild dysostosis multiplex. He also presents frequent upper respiratory infections, bilateral hearing loss, sleep disturbances, progressive neurologic deterioration and behavioral problems. He is compound heterozygous for the NAGLU gene (c.503G˃A; p.Trp168Ter/ c.3G˃A; p.met1?). One of the mutation was described in two patients before. A novel pathogenic variant was detected.
