Inclusion Criteria:  Subject completed participation in the primary study, XPF-009-301. A subject who withdraws from the primary study due to meeting protocol-specified worsening criteria will be considered as having completed participation in the primary study. The caregiver is willing and able to be compliant with diary completion, visit schedule, and study drug administration. Subject's caregiver achieved a minimum of 85% compliance with daily diary completion during 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 study XPF-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 those resulting from KCNQ2-DEE, present at screening/baseline that, in the opinion of the investigator, 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 the disallowed medications during the study. Any change in cardiac rhythm or atrioventricular conduction in the primary study that, in the investigator's opinion, is a significant risk to subject safety.