Inclusion Criteria:

• Has a documented Gain of Function SCN2A variant confirmed through genetic testing.
• Has onset of seizures prior to 3 months of age.
• Seizure frequency of 4 or more countable motor seizures per 28-day during the Baseline Observation Period.

Exclusion Criteria:

• Has any clinically significant or known pathogenic genetic variant other than in the SCN2A gene, or a genetic variant that may explain or contribute to the participant's epilepsy and/or developmental disorder.
• Has bone, spine (eg, kyphosis, scoliosis), bleeding, or other disorder.
• Has received any experimental or investigational drug, device, or other therapy within 30 days or 5 half-lives (whichever is longer) prior to Screening, including any prior use of gene therapy.
• Is currently pregnant or breastfeeding or is planning to become pregnant during the clinical trial.