Inclusion Criteria: * Age six months to 2 years of age at day of vector infusion. For those \<1 year of age they must have been ≥37 weeks gestational age at the time of birth and without other conditions/comorbidities that in the opinion of the Investigator may interfere with the interpretation of study results. * Confirmed diagnosis of propionic acidemia with biallelic PCCA gene mutations based on molecular genetic testing. * Study participants must have a diagnosis of neonatal-onset propionic acidemia with a documented episode of decompensation that can include any of the following findings: lethargy, poor feeding, irritability, vomiting, encephalopathy, respiratory failure, seizures, coma, metabolic acidosis, lactic acidosis, ketonuria, hypoglycemia, hyperammonemia, and cytopenias or history of recurrent hospitalizations. * Parents or legal guardians of study participants must agree to comply in good faith with the conditions of the study, including attending all of the required baseline and follow-up assessments, and parents or legal guardians must give consent for their child's participation. Exclusion Criteria: * Hemoglobin \<10 g/dl * Platelet count \< 100,000 per mm3 * Liver Enzyme ALT/AST \>2.5 ULN * Direct Bilirubin \> 1.5 * Active viral infection (includes HIV or serology positive for hepatitis B or C). * Previous liver transplant * Subjects with active decompensation as demonstrated by a pH \< 7.3, bicarbonate \< 15 mmol/L, NH3 \> 75 mcmol/L, lactate \> 2.5 mmol/L, urine ketones * Previously received gene therapy or messenger ribonucleic acid (mRNA) treatments for PA. * Grade 3 or 4 heart failure according to the Modified Ross Heart Failure Classification for Children or the New York Heart Association Classification. * Family does not want to disclose patient's study participation with primary care physician and other medical providers.