<qa>  What is the link between RTT clinical features and its genetic basis?Effective treatments for RTT are currently lacking. There is also inadequate information about the link between RTT 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. 