The purpose of this study is to advance understanding of the natural history of Rett syndrome (RTT), MECP2-duplication disorder (MECP2 Dup), RTT-related disorders including CDKL5, FOXG1, and individuals with MECP2 mutations who do not have RTT including the range of clinical involvement and to correlate genotype-phenotype over a broad spectrum of phenotypes. While much has been learned about RTT, improvements are required in understanding the role of factors such as X chromosome inactivation, genetic background, and others including the environment, on the great variability observed even between individuals with the same MECP2 mutation. These data will be essential to the development and conduct of clinical trials that are anticipated from ongoing studies in animal models for RTT. This study will not include clinical trials, but should set the stage for such trials and other translational research projects (e.g., development of biomarkers).