Phase I/II/III Gene Transfer Clinical Trial of scAAV9.U1a.hSGSH

Inclusion Criteria: * Diagnosis of MPS IIIA confirmed by the following methods: * No detectable or significantly reduced SGSH enzyme activity by leukocyte assay, and * Genomic DNA analysis demonstrating homozygous or compound heterozygous mutations in the SGSH gene (based upon review of documented results from a qualified laboratory, and with confirmation with Medical Monitor) * Age: * For Cohort 1-3: From birth (participating sites in USA and Australia) OR 6 months (participating sites in Spain) to 2 years of age with no BSITD-III Cognitive Development Quotient (DQ) requirement, or older than 2 years with a BSITD-III Cognitive DQ of 60 or above (participating sites globally). * For Cohort 4 (participating sites in Spain): 3 months to ≤ 2 years of age with no BSITD-III Cognitive DQ requirement or \> 2 years of age with a BSITD-III Cognitive DQ ≥ 60 (n = up to 6). Up to 2 additional subjects \> 2 years and ≤ 5 years of age with a BSITD-III Cognitive DQ \< 60 may also be enrolled. •Subjects must be ≥ 6 months of age before UX111 administration. However, subjects may be consented and initiate relevant Screening Procedures and IM treatment \< 6 months of age. Refer to Section 8.2 for relevant screening procedures •For children ≤ 24 months chronological age who were born prematurely, defined as born at \< 36 weeks gestational age, the corrected gestational age must be used for determining inclusion •The BSITD-III Cognitive DQ is assessed during the onsite Screening visi…