An Open-Label Extension of the Study XEN496 (Ezogabine) in Children With KCNQ2-DEE

Inclusion Criteria:

• Subject completed participation in the primary study, XPF-009-301. A subject whowithdraws from the primary study due to meeting protocol-specified worseningcriteria will be considered as having completed participation in the primary study.

• The caregiver is willing and able to be compliant with diary completion, visitschedule, and study drug administration.

• Subject's caregiver achieved a minimum of 85% compliance with daily diary completionduring both baseline and the double-blind period of the primary study.

Exclusion Criteria:

• Any adverse event(s) or serious adverse event(s) during the primary studyXPF-009-301, which in the opinion of the investigator and sponsor's medical monitor,would preclude the subject's entry into the OLE study.

• A clinically significant condition or illness, or symptoms other than thoseresulting from KCNQ2-DEE, present at screening/baseline that, in the opinion of theinvestigator, would pose a risk to the subject if s/he were to enter the study.

• Any conditions that were specified as exclusion criteria in the primary study,XPF-009-301.

• It is anticipated that the subject will require treatment with at least 1 of thedisallowed medications during the study.

• Any change in cardiac rhythm or atrioventricular conduction in the primary studythat, in the investigator's opinion, is a significant risk to subject safety.