These materials will be stored at a central repository (Greenwood Genetics Laboratory). The main purpose of these samples is to serve as durable materials for future analyses, however, a set of defined analyses will be performed on all samples. For the samples collected in the Rett syndrome cohort, investigators will perform X-chromosome inactivation studies and evaluate common polymorphisms in Brain derived neurotrophic factor (BDNF) and determine the contribution of these known factors to disease severity. For MECP2 Dup cohort investigators will characterize inflammatory markers in the plasma and correlate these with clinical features. Also for MECP2 Dup cohort investigators will perform detailed genomic breakpoint and gene content analysis and correlate this with disease severity. Similar analysis of genomic breakpoints and gene content will be performed on people with FOXG1 Duplications. Finally, in a pilot study, investigators will perform metabolic profiling on people from all disorders and evaluate for metabolic features correlated with disease severity, and metabolic features common or unique between these disorders. This work will provide a durable resource for future analysis, extend understanding of genotype/phenotype correlations, identify other biological factors contributing to disease severity, as well as provide the framework for the development of biomarkers of disease state and severity.