• Participants who are characterized as having Lennox-Gastaut Syndrome (LGS) must fulfill all of the following criteria:

‣ Onset of seizures at ≤8 years old

⁃ History of tonic/tonic-atonic seizures plus at least 1 of the following seizure type(s): atypical absence, atonic, myoclonic, focal impaired awareness, generalized tonic-clonic, nonconvulsive status epilepticus, or epileptic spasms

⁃ Presence of developmental plateauing or regression

⁃ History of electroencephalogram (EEG) showing generalized slow (\<2.5 Hertz \[Hz\]) spike-and-wave complexes

• Participants who are characterized as having DEE (Other) must fulfill all of the following criteria:

‣ Does not meet criteria for LGS

⁃ Onset of seizures at ≤5 years old

⁃ Presence of developmental plateauing or regression

⁃ History of multiple seizure types

⁃ History of interictal EEG background showing diffuse or multifocal slowing (with or without epileptiform activity)

• The participant has a current occurrence of at least 1 of the following countable motor seizure types: generalized tonic-clonic, tonic (bilateral), clonic (bilateral), atonic (bilateral) with truncal/leg involvement, focal motor (including hemiclonic), and focal to bilateral tonic-clonic.

• The participant has demonstrated an average of at least 4 countable motor seizures per month for each of the 3 months prior to Screening.

• The participant has been taking 1 to 4 antiseizure medications (ASMs) at a stable dose for at least 4 weeks prior to Screening.

• The participant, parent, or caregiver is willing and able (in the judgment of the investigator) to comply with completion of the diaries throughout the study.

• The participant must be willing and able to provide written informed consent; in instances where the participant is unable to provide consent, an appropriate legal representative.