• Male and female participants 2 years and older at time of consent.

• Participant or parent/legally authorized representative (LAR) willing and able to provide written informed consent, assent (if applicable) prior to initiation of any study related procedures.

• Clinical diagnosis of DS. Participants must have seizures which are not completely controlled by AEDs with the following criteria:

‣ Onset of seizures prior to 18 months of age,

⁃ Normal development at onset,

⁃ History of at least one type of countable motor seizure (CMS),

⁃ Brain MRI without cortical malformation (not including mild atrophy associated with the natural progression of DS),

⁃ Genetic mutation of the SCN1A gene must be documented.