**Study of Fenfluramine Hydrochloride in Rett Syndrome** This research is checking if a medicine called fenfluramine hydrochloride helps people with Rett Syndrome, a rare disorder affecting brain development. Rett Syndrome mostly affects girls and leads to severe mental and physical problems. The study compares this medicine to a fake treatment, called a placebo, to see if it works better. **Eligibility:** - Participants must have typical Rett Syndrome and a specific gene change. - Age limit: 5 to 35 years old. - Participants need a caregiver to help with the study. **Exclusion:** - Recent cancer history or heart/lung problems. - Taking more than four medicines for seizures. - Serious health issues that could affect the study. **Key Points:** - Participants will receive either the medicine or a placebo. - Caregivers will help with assessments throughout the study. - Participants' health will be monitored carefully. This study is important for discovering better treatments for Rett Syndrome and might help improve the quality of life for those affected. It’s crucial to meet all the conditions for safety and accurate results.