 Rett Syndrome RTT is a genetic brain disorder that occurs almost exclusively in females and is usually caused by a change mutation in the gene MECP2. The disorder is characterized by multiple developmental problems, as well as behavioral features, such as repetitive stereotypic hand movements, including hand washing, wringing, and tapping. While there is no cure for RTT, recent advances in the understanding of the disease suggest that the development of new, effective therapies is promising. This study will gather information on the genetic defects that cause RTT, the physical expressions of these defects, and disease progression. In turn, this may direct the development of future treatments. Expanded studies include individuals with MECP2 Duplication disorder, and RTT-related disorders including individuals with MECP2 mutations, but not meeting obligatory criteria for the diagnosis of RTT and individuals with mutations in CDKL5 and FOXG1 some of whom meet criteria for atypical RTT. 