• Patient (male or female) aged ≤16 years at time of written informed consent. For the dose escalation phase patients must be aged 6-16, for the dose expansion phase patients must be aged 0-16 (at the time of written informed consent).

• OTC deficiency confirmed via enzymatic or molecular analysis. This may include identification of pathogenic mutations or liver OTC activity that is \<20% of normal activity.

• Patient has severe disease defined by reduced protein allowance and prescribed at least one ammonia scavenger drug.

• Patient (if capable of signing) and parents or legal representative have signed a written informed consent form.

• Females of childbearing potential must have a negative pregnancy test in serum or urine at the screening and Day 0 infusion visits, and use an adequate contraception method from the screening visit until 4 weeks after the first negative plasma sample monitoring vector genomes copies or the week 52 visit, whatever comes first.

• Sexually active boys must use an adequate contraception method (abstinence or use of condom with spermicide) from at least 14 days prior to the infusion and until 4 weeks after the first negative plasma sample monitoring vector genomes copies or the week 52 visit, whatever comes first.

• Patient's ammonia level at baseline visit (pre-gene therapy infusion) is \<100µmol/L and is within the range of historical ammonia levels obtained when the patient was clinically stable.

• Patient has been on a stable dose of ammonia scavenger and stable protein allowance for the last 4 weeks at the baseline visit.

• Patient is willing to commit to an additional 4 years of long-term safety follow-up.