Natural History Study to Characterise the Course of Disease Progression in Participants With Mucopolysaccharidosis Type IIIB

Inclusion Criteria:

A participant must meet all of the following inclusion criteria to be eligible for this
study:

1. The participant has a definitive diagnosis of MPS IIIB, as determined by either of the
following:

1. Documented deficiency in alpha-N-acetyl-glucosaminidase (NAGLU) enzyme activity
or

2. Documented functionally-relevant mutations in both alleles of the NAGLU gene.

2. The participant is at least 1 year of age (biological age).

3. The participant or the participant's parent provides informed consent.

4. The participant is willing and able to comply with protocol requirements to the extent
that may be expected of a participant with cognitive impairment.

In addition to the eligibility criteria above, a participant must meet all of the following
criteria for Component 2:

1. The participant meets criterion a or criterion b below.

a. The participant is considered to be at risk of rapid disease progression based on
at least 1 of the following criteria:

i. The participant has documented mutations of the NAGLU gene that are reported to be
linked to rapid disease progression (for example, disease onset before 6 years of
age), or

ii. The participant has a sibling, or other first- or second-degree relative with
rapidly progressing MPS IIIB (for example, disease onset before 6 years of age).

b. The participant had disease onset prior to 6 years of age (biological age), as
defined by:

i. Cognitive delay evaluated by Bayley Scales of Infant Development, Third Edition
(BSID-III) or Kaufman Assessment Battery for Children, Second Edition (KABC-II), or

ii. Language delay, plateauing, or regression of language skills as determined by the
Investigator (for example, participant uses isolated words, associated words such as
2-word combinations, sentences, poor or reduced language, and/or difficult to
understand).

2. The participant has an age equivalent of ≥1 year on the Vineland Adaptive Behavior
Scales, Second Edition (Vineland II).

Exclusion Criteria:

A participant who meets any of the following exclusion criteria will be ineligible for this
study:

1. The participant has visual or hearing impairments sufficient to preclude cooperation
with neurodevelopmental testing.

2. The participant has a history of poorly-controlled seizure disorder.

3. The participant is currently receiving medication, which, in the Investigator's
opinion, would be likely to substantially confound interpretation of the results (for
example, the participant has been on the current dose of psychotropic medication for
less than 3 months).

4. The participant is receiving a newly increased dose of melatonin (for example, less
than 3 months on current dose).

5. The participant has previously received an investigational therapy for MPS IIIB (with
the exception of high dose Genistein >150 milligram/kilogram (mg/kg)/day, which will
require a minimum of 3 months wash-out before entering the study) or has had
hematopoietic stem cell transplant (HSCT).

6. The participant has any other prior or ongoing medical condition that may present a
safety risk, interfere with study compliance, or confound data interpretation.