A Treatment Study of Mucopolysaccharidosis Type IIIB

Inclusion Criteria: Individuals eligible to participate in Part 1 of this study must meet all of the following criteria: * Has deficient NAGLU enzyme activity at Screening. Blood for NAGLU enzyme activity will be collected and analyzed centrally. * Is ≥ 1 and \< 11 years of age (at least 1 of the 3 subjects in Part 1 must be ≥ 1 and \< 6 years of age) * Has presented with signs/symptoms consistent with MPS IIIB; for individuals who have not presented with signs/symptoms of disease (eg, siblings of known patients), the determination of eligibility will be at the discretion of the BioMarin medical monitor in conjunction with the site investigator. * Written informed consent from parent or legal guardian and assent from subject, if required * Has the ability to comply with protocol requirements, in the opinion of the investigator Individuals eligible to participate in Part 2 of this study must meet all of the following criteria: * Participated in and met protocol requirements for transitioning from Study 250-901 or participated in Part 1 of Study 250-201 * Written informed consent from parent or legal guardian and assent from subject, if required Exclusion Criteria: Individuals who meet any of the following exclusion criteria are ineligible to participate in Part 1 of the study: * Has received stem cell, gene therapy or ERT for MPS IIIB * Has contraindications for neurosurgery (eg, congenital heart disease, severe respiratory impairment, or clotting abnormalities) * Has contraindications for MRI scans (eg, cardiac pacemaker, metal fragment or chip in the eye, or aneurysm clip in the brain) * Has a history of poorly controlled seizure disorder * Is prone to complications from intraventricular drug administration, including patients with hydrocephalus or ventricular shunts * Has received any investigational medication within 30 days prior to the Baseline visit or is scheduled to receive any investigational drug during the course of the study * Has a medical condition or extenuating circumstance that, in the opinion of the investigator, might compromise the subject's ability to comply with protocol requirements, the subject's well-being or safety, or the interpretability of the subject's clinical data. * Is pregnant at any time during the study Individuals who meet any of the following exclusion criteria are ineligible to participate in Part 2 of this study: * Has received stem cell, gene therapy or ERT for MPS IIIB * Has contraindications for neurosurgery (eg, congenital heart disease, severe respiratory impairment, or clotting abnormalities) * Has contraindications for MRI scans (eg, cardiac pacemaker, metal fragment or chip in the eye, or aneurysm clip in the brain) * Is prone to complications from intraventricular drug administration, including patients with hydrocephalus or ventricular shunts * Has received any investigational medication within 30 days prior to the Baseline visit or is scheduled to receive any investigational drug during the course of the study * Has a medical condition or extenuating circumstance that, in the opinion of the investigator, might compromise the subject's ability to comply with protocol requirements, the subject's well-being or safety, or the interpretability of the subject's clinical data. * Is pregnant at any time during the study