Evaluate the clinical progression in patients with MPS IIIA who are untreated with any investigational product and to obtain standardized assessments: neurocognitive, behavioral, sleep-wake habits and effect of MPS IIIA on the quality of life of patients and their families.

This is a multicenter, multinational, longitudinal, observational study in children aged up to and including 9 years, who have been diagnosed with MPS IIIA. The study will detail the natural course of MPS IIIA via standardized clinical, biochemical, neurocognitive, developmental, behavioral and imaging measures. This information is anticipated to inform the design and interpretation of future interventional studies.

Inclusion Criteria:

• Documented MPS IIIA diagnosis
• Children up to and including 9 years of age
• The patient is sufficiently able, in the opinion of the Investigator, to adhere to the study visit schedule and other protocol requirements
• The patient's parent(s) or legal guardian(s) has signed written informed consent, according to the local regulations and after all relevant aspects of the -study have been explained and discussed

Exclusion Criteria:

• The patient is participating in a clinical trial of any potential disease-modifying investigational medicinal product or taking high dose (>100 mg/kg/day) synthetic genistein (patients on low dose or naturally derived genistein can be included in this study).
• The patient has received a hematopoietic stem cell or bone marrow transplant or gene therapy.
• The patient has received enzyme replacement therapy in the last 6 months.
• Homozygous or compound heterozygous for the S298P mutation or the investigator and/or trial steering committee considers the patient not to have the classical severe form of MPS IIIA.
• Individuals with rare and unrelated serious comorbidities e.g. Down syndrome, intraventricular hemorrhage in the new-born period, or extreme low birth weight (<1500 grams).
• Visual or hearing impairment sufficient, in the clinical judgment of the investigator, to preclude cooperation with neurodevelopmental testing. Use of hearing aids is permitted.

Subjects diagnosed with MPS IIIA