<qa>  How long will the first visit last?Participants in this observational study will be recruited from the four sites at which the study is being conducted, as well as through the Rare Disease Clinical Research Network and the International Rett Syndrome Association IRSA. Prior to study entry, potential participants are expected to be tested for a mutation in the MECP2 gene. No treatment will be administered at any time during this study. Study visits will occur every 6 months until the child is 6 years old and once a year thereafter. At each study visit, participants will be examined to assess physical characteristics of the disorder, such as motor behavior and disease severity. Additionally, participants will complete questionnaires about medical history, contact information, and quality of life. The first visit will last approximately 1.5 hours, and every subsequent visit will last approximately 1 hour. 