Inclusion Criteria: * Confirmed diagnosis of MPSIIIB by both of the following two methods: * No detectable or significantly reduced NAGLU enzyme activity by plasma. * Genomic DNA analysis demonstrating homozygous or compound heterozygous mutations in the NAGLU gene * Age: From Birth to 2 years or children older than 2 years with a minimum cognitive Development Quotient (DQ) of 60 or above (calculated by Bayley Scales of Infant and Toddler Development - Third Edition) Exclusion Criteria: * Inability to participate in the clinical evaluation as determined by Principal Investigator * Identification of two nonsense or null variants on genetic testing of the NAGLU gene * Has evidence of an attenuated phenotype of MPS IIIB * Presence of a concomitant medical condition that precludes lumbar puncture or use of anesthetics * Active viral infection based on clinical observations as infections by Adenoviruses, Epstein-Barr Virus, Cytomegalovirus, Respiratory Syncytial Virus * Concomitant illness or requirement for chronic drug treatment that in the opinion of the PI creates unnecessary risks for gene transfer , or precludes the child from participating in the protocol assessments and follow up as autoimmune diseases requiring immunosuppression, such as juvenile rheumatoid arthritis or idiopathic thrombocytopenia purpura * Subjects with total anti-AAV9 antibody titers ≥ 1:100 as determined by ELISA binding immunoassay * Subjects with a positive response for the ELISPOT for T-cell responses to AAV9 * Serology consistent with exposure to HIV, or serology consistent with active hepatitis B or C infection * Bleeding disorder or any other medical condition or circumstance in which a lumbar puncture (for collection of CSF) is contraindicated according to local institutional policy * Visual or hearing impairment sufficient to preclude cooperation with neurodevelopmental testing * Uncontrolled seizure disorder * Any item (braces, etc.) which would exclude the subject from being able to undergo MRI according to local institutional policy * Any other situation that precludes the subject from undergoing procedures required in this study * Subjects with cardiomyopathy or significant congenital heart abnormalities * The presence of significant non-MPS IlIB related CNS impairment or behavioral disturbances that would confound the scientific rigor or interpretation of results of the study * Abnormal laboratory values Grade 2 or higher as defined in CTCAE v4.0 for GGT, total bilirubin, creatinine, hemoglobin, WBC count, platelet count, PT and aPTT * Female participant who is pregnant or demonstrates a positive urine or serum result at screening assessment (if applicable). * Any vaccination with viral attenuated vaccines less than 30 days prior to the scheduled date of treatment (and use of prednisolone) * Previous treatment by Haematopoietic Stem Cell transplantation * Previous participation in a gene/cell therapy or ERT clinical trial