NAGLU and lysosomal storage disease: Mucopolysaccharidosis type IIIB (MPS IIIB or Sanfilippo syndrome B, OMIM #252920) is an autosomal recessive lysosomal storage disorder (LSD) caused by mutations in the gene encoding the lysosomal hydrolase, N-alpha-acetylglucosaminidase (NAGLU or NAG; E.C. 3.2.1.50).