Among other things, the present invention provides methods and compositions of treating Sanfilippo syndrome type B (Sanfilippo B) by, e.g., intrathecal (IT) administration of a Naglu protein. A suitable Naglu protein can be a recombinant, gene-activated or natural protein. In some embodiments, a suitable Naglu protein is a recombinant Naglu protein. In some embodiments, a recombinant Naglu protein is a fusion protein containing a Naglu domain and a lysosomal targeting moiety. In some embodiments, the lysosomal targeting domain is an IGF-II moiety.