Inclusion Criteria: Male or female, ≥18 months to \<8 years of age; Has diagnosis of developmental encephalopathy due to an STXBP1 mutation with confirmation of a pathogenic or likely pathogenic STXBP1 gene mutation. Has a legally authorized representative (LAR) willing and able to complete the informed consent process, willing to comply with trial procedures, and able to travel for repeat visits. Is stable on any medication regimens (if being administered to control the signs and symptoms of underlying disease) for at least 6 weeks prior to trial entry and expected to be stable for at least 12 weeks post-CAP-002 administration. Exclusion Criteria: History of prior gene therapy; Treatment with antisense oligonucleotide therapy within 6 months; Presence of a confirmed mutation in a gene other than STXBP1 that is known to contribute to a neurodevelopmental disability or epilepsy; Has presence of a significant non-STXBP1-related central nervous impairment/behavioral disturbance that would confound the scientific rigor or interpretation of results of the trial; History of prematurity (defined as gestational age \<35 weeks), history of low birth weight (\<2.5 kg) and/or intra-uterine growth restriction, significant interventricular hemorrhage, structural brain deficit, or congenital heart disease; Known contraindication to immunosuppression or other protocol-defined medications, including but not limited to corticosteroids or PPIs; Clinically significant abnormalities in safety lab tests, vital signs; Other illnesses or medications that may affect the interpretation of the study results; Positive anti-capsid antibody test result.