• 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 visit, for subjects who require it •The age of the child on the date of the Screening BSITD-III assessment is used to determine the requirement for the BSITD-III Cognitive DQ score.

• Cohort 4 only: Vaccination status based on age according to country-specific guidelines that is up to date 30 days prior to Enrollment as verified by documentation from the subject's primary care physician, and willing to defer vaccines through 6 months after completion of the subject's IM medication, or longer per Principal Investigator (PI) judgment. Emergency use authorization or conditional marketing authorization of coronavirus disease (COVID) vaccines is included unless there is an accepted medical exemption.