The aim of this study is to evaluate the safety and tolerability of triheptanoin in participants with Rett syndrome using laboratory values, electrocardiogram, rate of adverse events (AE), and physical exam. This study also seeks to evaluate the efficacy of UX007 (triheptanoin) in improving overall seizure frequency, dystonia severity, and quality of life. Participants who are eligible will take triheptanoin daily. Participation in this study will last up to 8.5 months.