Inclusion Criteria:
1. The child patient has to be ≥2 years old and \< 8 years old, or a head circumference big enough for surgery.
2. Historical diagnosis of AADC deficiency with clinical symptoms consistency, AND confirmed by one of the lab tests: (1) CSF neurotransmitter profile demonstrating reduced HVA and 5-HIAA, and elevated L-Dopa concentrations; (2) Plasma AADC activity less than or equal to 5 pmol/min/mL, AND with Molecular genetic confirmation of homozygous or compound heterozygous mutation of IVS6+4A\>T in DDC.
3. Motor development at baseline ≤3 months, and Failed to benefit from standard medical therapy (dopamine agonists, monoamine oxidase inhibitor or related form of Vitamin B6) at discretion of investigators.
4. Stable medication regimen for treatment of AADC deficiency: (i.e. no new medications introduced for at least 6 months, and no existing medication dose changes for at least 3 months prior to Baseline).
5. Parent(s)/legal guardian(s) with custody of subject must give their consent for subject to enroll in the study
6. Parent(s)/legal guardian(s) of the subject must agree to comply with the requirements of the study, including providing disease information and support disease assessment of symptoms.
Exclusion Criteria:
1. Intracranial neoplasm or any structural brain abnormality or lesion (e.g., severe brain atrophy, white matter degenerative changes), which, in the opinion of the study investigators, would confer excessive risk and/or inadequate potential for benefit.
2. Presence of other significant medical or neurological conditions that would create an unacceptable operative or anesthetic risk (including congenital heart disease, respiratory disease with home oxygen requirement, history of serious anesthesia complications during previous elective procedures, history of cardiorespiratory arrest), liver or renal failure, malignancy, or HIV positive.
3. Severe coagulopathy, or need for ongoing anticoagulant therapy.
4. clinically active infection or with severe infection within 12 weeks before screening (e.g. adenovirus or herpes virus, pneumonia, sepsis, central nervous system infection).
5. Previous stereotactic neurosurgery, or any gene/cell therapy.
6. Received live vaccination within 4 weeks.
7. Patients with anti-AAV9 neutralizing antibody titer over 1,200 folds.
8. Contraindication to sedation during surgery or imaging studies (PET or MRI).
