At the present time, effective treatments for RTT, MECP2 Dup, or Rett-related disorders are lacking. Substantial progress has been made in RTT over the past eleven years such that this study represents a narrowing of focus to mutations or duplications of the MECP2 gene and related disorders, including those with phenotypic overlap. Understanding of RTT has advanced remarkably well through the Rett Syndrome Natural History Clinical Protocol (NHS) and correspondingly advancement in the basic science realm has moved forward with equivalent success. Thus, progress in clinical and basic science has led to the establishment of clinical trials and other translational studies that hold promise for additional clinical trials in future. In the process, however, additional MECP2- and RTT-related disorders that were unknown at the time the original proposal have been identified. In addition, substantial clinical variability in individuals with RTT that cannot be explained by differences in mutations alone must be explored further. In fact, variability among individuals with identical mutations has led to the search for additional explanations. At the time of the initial application (2002), just three years after the identification of the gene, MECP2, as the molecular link to RTT, the variation in clinical disorders related to MECP2 mutations or to the related but quite different MECP2 Dup were unknown. Each disorder is characterized by significant neurodevelopmental features related either to alterations in the MECP2 gene or related to phenotypes closely resembling those seen in individuals with RTT. Further, the phenotypic overlap with RTT due to mutations in CDKL5 and FOXG1 was also unexplored. This new study will build on the substantial progress made in understanding both classic and variant RTT and to add these related disorders, MECP2 Dup and the Rett-related disorders including CDKL5, FOXG1, and individuals with MECP2 mutations who do not have RTT. A comprehensive clinical research program will be performed including clinical, neurophysiologic, and molecular and biochemical markers across these different, but related disorders. This protocol will address the natural history components only and will serve as the basis for other study protocols including the neurophysiologic and biomarker studies. Thereby, these studies will represent a continuing pathway to focus and inform not only the ongoing but also the emerging clinical trials.