Rett Syndrome Atypical

The Rett Syndrome Registry is a longitudinal observational study of individuals with MECP2 mutations and a diagnosis of Rett syndrome. Designed together with the IRSF Rett Syndrome Center of Excellence Network medical directors, this study collects data on the signs and symptoms of Rett syndrome as reported by the Rett syndrome experts and by the caregivers of individuals with Rett syndrome. This study will be used to develop consensus based guidelines for the care of your loved ones with Rett syndrome and to facilitate the development of better clinical trials and other aspects of the drug development path for Rett syndrome.

The goal of this observational study is to identify candidate biomarkers in individuals with Rett Syndrome (RTT). The main questions it aims to answer are: * Do these biomarkers change during clinical changes in individuals with RTT? * Are biomarkers stable over time in clinically stable individuals? * Do these biomarkers correlate with severity of RTT? Participants will be asked to undergo an electroencephalogram (EEG) with measurements of Evoked Potentials (EP) to measure electrical activity in the brain. Researchers will compare findings in individuals with RTT to those in typically developing individuals to see if there are differences between the two groups.