Inclusion Criteria: 1. Written informed consent of a legally authorized guardian(s) 2. Age at baseline ≥3 months and ≤24 months 3. Normal cognitive function or mild cognitive deterioration (subject has a Development Quotient (DQ) score ≥80) at baseline as determined by the Bayley Scale of Infant Development-third edition (BSID-III), cognitive domain) 4. Sibling or relative of known MPS IIIA patients with rapidly progressing phenotype, or genotype associated with rapidly progressing phenotype, or presence of somatic features predictive of rapid progression 5. SGSH activity ≤10% of the Lower Limit of Normal as measured in leukocytes, plus either (1) a normal enzyme activity level of at least one other sulfatase (to rule out multiple sulfatase deficiency) as measured in leukocytes or (2) two documented mutations in the SGSH gene. 6. Medically stable and able to accommodate the protocol requirements, including travel without placing an undue burden on the patient/patient's family, as determined by the CI. Exclusion Criteria: 1. The subject has received stem cell, gene therapy or enzyme replacement therapy (any route of administration) 2. Subject currently enrolled in other interventional clinical trials. 3. Contraindications for MRI scans. 4. The subject has a history of poorly controlled seizures. 5. Homozygous or compound heterozygous for the S298P mutation or any other mutation known to be associated to slow-progressing phenotype. 6. The subject is currently receiving psychotropic or other medications which, in the CI's opinion, would be likely to substantially confound test results. 7. The subject has received any investigational medicinal product (including Genistein) within 30 days prior to the Baseline visit or is scheduled to receive any investigational medicinal product during the course of the study. 8. Documented Human Immunodeficiency Virus (HIV) infection (positive HIV RNA and/or anti-p24 antibodies). 9. Malignant neoplasia (except local skin cancer) or a documented history of hereditary cancer syndrome. Subjects with a prior successfully treated malignancy and a sufficient follow-up to exclude recurrence (based on oncologist opinion) can be included after discussion and approval by the Medical Monitor. 10. Myelodysplasia, cytogenetic alterations characteristic of myelodysplastic syndrome and acute myeloid leukaemia, or other serious haematological disorders. 11. The subject has a medical condition or extenuating circumstance that, in the opinion of the CI, 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. 12. Visual or hearing impairment sufficient to preclude cooperation with neurodevelopmental testing. 13. Severe behavioural disturbances due to reasons other than MPS IIIA and likely to interfere with protocol compliance, as determined by the CI. 14. Known sensitivity to busulfan. 15. The receipt of live vaccinations within 30 days prior to study start.