This is a natural history study for children up to 18 years of age who have been diagnosed with Mucopolysaccharidosis Type IIIB (MPS IIIB, also known as Sanfilippo Syndrome Type B). Mucopolysaccharidosis type IIIB is a severe neurodegenerative disorder. The information gathered from this trial may help inform the design and interpretation of subsequent interventional studies. No clinical intervention or study drug is provided by Allievex in this study.

Inclusion Criteria:

• Have deficient NAGLU enzyme activity at Baseline. Blood for NAGLU enzyme activity will be collected and analyzed centrally.
• Is up to 18 years of age
• 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

Exclusion Criteria:

• Has another neurological illness that may have caused cognitive decline (e.g., trauma, meningitis, or hemorrhage) before study entry
• Has received stem cell, gene therapy, or enzyme replacement therapy for MPS IIIB
• 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 wellbeing or safety, or the interpretability of the subject's clinical data.
• Is currently participating in another natural history study

Males and Females with a documented diagnosis of MPSIIIB