EPI-743 in Leigh syndrome participants that participated in previous EPI743-12-002 (NCT01721733) study.

To monitor the long-term safety and neurodevelopmental effects of EPI-743 on children who complete the EPI743-12-002 placebo-controlled trial.

Inclusion Criteria:

1. Diagnosis of Leigh syndrome with genetic confirmation
2. Completion of EPI743-12-002 protocol and initiation of treatment within 14 days of Month 12 visit in EPI743-12-002 study
3. Participant or participant's guardian able to consent and comply with protocol requirements
4. Continued abstention from supplements excluded in EPI743-12-002 study
5. Botox® is allowed if approved by the sponsor

Exclusion Criteria:

1. Allergy to EPI-743 or sesame oil
2. Allergy to vitamin E
3. Clinical history of bleeding or abnormal baseline prothrombin time (PT)/partial thromboplastin time (PTT)
4. Hepatic insufficiency with liver function tests (LFTs) greater than two times upper limit of normal
5. Renal insufficiency requiring dialysis
6. End-stage cardiac failure
7. Fat malabsorption syndromes precluding drug absorption
8. Use of anticoagulant medications
9. Participation in other clinical research studies/taking other experimental agents
10. Participation in elective procedures that required sedation