RTT is a brain disorder that causes problems with childhood development. It is usually caused by an abnormality (mutation) in the gene MECP2. RTT can cause severe impairments in movement and communication skills, including talking and social interaction. The first signs of RTT include loss of acquired speech and loss of purposeful hand use for activities such as eating or playing. Individuals may also develop abnormal walking, repetitive hand movements, such as clapping or wringing, and abnormal breathing while awake.

Effective treatments for RTT are currently lacking. There is also inadequate information about the link between RTT's clinical features and its genetic basis. In order to prepare for future clinical trials that may lead to effective therapies, it is important to collect accurate information about the characteristics of RTT and the pattern of disease progression. This study will gather historical and physical examination data to establish phenotype-genotype correlations. Data on survival and quality of life in females with RTT and males with MECP2 gene mutations will also be evaluated.

Participants in this observational study will be recruited from the three 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 12 years old and once a year for 5 years thereafter. At each study visit, participants will be examined to assess physical effects of the disease, 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.