• Added the rationale for defining hyperammonemia as an ammonia level ≥ 100 µmol/L • Increased the dosing interval between subjects in Cohort 1 and Cohort 2 from 7 days to 14 days and added the rationale for this change • Reversed the order of the secondary endpoints; designated the rate of ureagenesis as the main efficacy parameter to identify the DTX301 OBD • Updated the DTX301 dose levels for Dose 1 and Dose 2 • Decreased the number of inpatient visits • Changed the timing of tapering or discontinuing ammonia scavenger medication to after the Week 12 and Week 24 Visits • Revised the study stopping criteria • Updated Inclusion Criterion 2 with specific examples of testing to confirm OTC deficiency • Added Inclusion Criterion 5 to ensure subjects had stable OTC deficiency before receiving DTX301 • Combined Inclusion Criterion 10 with Exclusion Criterion 13 to create one criterion related to or pregnancy • Revised assessment time points for the rate of ureagenesis due to the decrease in inpatient visits; added the option to assess the rate of ureagenesis at any outpatient visit if clinically indicated • Revised the criteria for starting corticosteroids for suspected vector-induced hepatitis • Decreased the number of neuropsychological evaluations • Added clinical chemistry assessments at Weeks 18 and 22 • Removed text specifying that subjects would receive a prescribed diet during each inpatient stay and that the subject’s prescribed diet may be adjusted

• Expanded text regarding tapering of ammonia scavenger medication • Added a requirement for plasma ammonia level to be < 100 µmol/L prior to administration of neuropsychological tests and [1-13C]sodium acetate • Added text regarding repeat liver function testing to inform use of corticosteroids for possible vector-induced hepatitis • Added assessment of anti-OTC antibodies • Updated the sample type for ammonia assessments to plasma

• Changed the timing of DMC review for each dose cohort from Week 6 to Week 12 • Added guidance for rescreening patients who fail screening • Added text to allow flexibility in the timing and sequence of assessments • Added text to specify that the Day 1 (predose) plasma ammonia level must be assessed within 12 hours or less before DTX301 administration • Added text to specify that, when a stopping rule is met, a substantial amendment needs to be approved by the regulatory authority to restart enrollment following DMC review • Added a definition of abstinence as a form of birth control • Deleted Inclusion Criterion 7 because tapering or discontinuing ammonia scavenger medications is not required • Corrected the anti-AAV8 neutralizing antibody titer in Exclusion Criterion 9 • Added assessment of liver function tests at the local laboratory (STAT samples) through Week 12 • Changed the ALT level for considering initiation of corticosteroids from 2.5 × ULN to > ULN and updated Exclusion Criterion 4 to reflect this change • Added additional assessments of cell-mediated immune response to AAV8 and OTC through Week 12 • Added text to allow subjects to rest during administration of neuropsychological tests and to explain the importance of assessing neuropsychological functioning long-term

• Added a reference to another AAV gene therapy study using a prophylactic corticosteroid regimen • Removed outdated information regarding the DTX301 manufacturing process • Incorporated changes detailed in Addendum A to Amendment 3 (dated 02 March 2017), which updated Figure 3-1 Study Design to align with protocol text • Incorporated changes detailed in Addendum B (UK-specific) to Amendment 3 (dated 21 April 2017), which specified the timing of dosing for subjects at sites in the UK • Added details regarding enrolling additional subjects in Cohort 3 and subjects in Cohort 4 • Added a brief summary of safety results from the study • Added a reference to another AAV gene therapy study that had a dosing interval of 1 day between subjects • Added a description of the prophylactic corticosteroid regimen for Cohort 4 • Extended the Screening Period from 28 days to 35 days • Updated the basis for adjusting or tapering ammonia scavenger medication to the Investigator’s clinical judgement based on review of the totality of a subject’s clinical and laboratory data • Updated instructions for dose preparation and administration of [1-13C]sodium acetate and added a reference to the ureagenesis manual • Added instructions for subject fasting before and after administration of [1 13C]sodium acetate • Updated the timing of DMC meetings to account for additional subjects in Cohort 3 and subjects in Cohort 4 • Removed text regarding finalization of the SAP prior to the start of the study • Updated the definition of the baseline rate of ureagenesis • Updated text regarding the presentation of other laboratory results and neuropsychological tests • Updated text regarding interim analysis to allow flexibility in the timing of the interim analysis and removed text stating that any interim analyses conducted will not bias the conduct of the study

• Clarified the age ranges for neonatal-onset OTC deficiency and late-onset OTC deficiency • Added guidance for monitoring ammonia levels, monitoring HAC, and modifying ammonia scavenger medication and protein-restricted diet • Added the term HAC and its definition • Updated the list of AAV8 clinical studies referenced in Section 1.2 • Removed details regarding the timing of dosing for subjects at sites in the UK, which was specific to Cohort 1 • Updated the timing of liver enzyme elevations following DTX301 administration based on study data • Added a reference to another AAV gene therapy study using a prophylactic corticosteroid regimen • Removed the definition of the baseline rate of ureagenesis • Removed “in the setting of tapering or discontinuing ammonia scavenger medications” from the exploratory endpoints for urinary orotic acid excretion and plasma glutamine and glutamate • Removed “weekly” from the exploratory endpoint for use of ammonia scavengers • Removed the exploratory endpoints for cell-mediated immune response to AAV8 and OTC • Updated the total sample size • Added text to allow abbreviated outpatient study visits to be completed by home health services where available, agreed upon by the Investigator, and allowed by local regulation • Expanded guidance for rescreening patients who fail screening • Updated text regarding the subject’s plasma ammonia level prior to administration of neuropsychological tests and [1-13C]sodium acetate to include “within the range of historical ammonia levels obtained when the subject was clinically stable” • Updated text, including Inclusion Criterion 5, regarding the subject’s plasma ammonia level prior to DTX301 administration to address subjects who historically maintain normal ammonia levels and subjects who historically do not have fully controlled ammonia levels with baseline management

(continued) • Updated Exclusion Criterion 1 to address subjects who historically maintain normal ammonia levels, subjects who historically do not have fully controlled ammonia levels with baseline management, and subjects who have signs and symptoms of hyperammonemia during the 4-week period preceding Day 0 • Added text to allow the rate of ureagenesis assessment to be repeated during Screening if discrepant with the subject’s clinical status and severity • Clarified details regarding plasma ammonia assessment prior to DTX301 administration • Added text to allow additional assessments of liver function tests, plasma ammonia, or other biomarkers at the Investigator’s discretion • Added spot ammonia assessment to additional study visits • Added text to explain that subjects will be re-educated on the risks of adjusting baseline treatment on their own without guidance from the Investigator • Revised instructions for modifying baseline treatment; baseline treatment cannot be changed during or within 2 weeks of corticosteroid administration and changes to ammonia scavenger medication and protein-restricted diet cannot occur at the same time • Added text to clarify that the rate of ureagenesis cannot be used to make decisions for modifying ammonia scavenger medication or protein-restricted diet • Added guidance for reinitiating ammonia scavenger medication and reattempting modification of baseline treatment • Expanded guidance for modifying protein-restricted diet • Updated Inclusion Criterion 6 to clarify that subjects must be receiving a daily stable dose of ammonia scavenger medication • Updated the number of medical personnel required to check [1-13C]sodium acetate dosing calculations • Updated text regarding treatment compliance to include ammonia scavenger medication and protein-restricted diet • Removed the assessment of vector genome in blood • Removed the ECG assessment from the Day 1 Visit prior to DTX301 administration

(continued) • Specified that, prior to initiating the prophylactic corticosteroid regimen (Cohort 4), the subject must be clinically and metabolically stable without intercurrent illness or receiving concomitant medications known to affect aminotransferase levels • Added guidance for ad hoc assessment of 24-hour plasma ammonia • Added text to specify that hospitalization due to HAC will be considered an SAE • Added text to specify that AEs will be assessed for relationship to corticosteroids in addition to the DTX301, [1-13C]sodium acetate, OTC deficiency, and hyperammonemia • Updated the CTCAE version from Version 4.03 to the most current version • Added a DMC meeting after completion of Week 12 for the first 3 subjects in Cohort 4 • Updated analysis of the rate of ureagenesis to include the relative percentage to normal healthy adults • Updated analysis of plasma ammonia to include time-normalized plasma ammonia • Decreased the frequency of viral shedding, AAV8 neutralizing antibody, and AAV8 binding antibody IgG assay assessments