SUM M ARY The purpose of this proposal is to build on the foundation of the Fragile X Online Registry With Accessible Research Database (FORWARD) and obtain more detailed longitudinal data describing the phenotype of children and young adults with fragile X syndrome (FXS) in order to better characterize the natural history of fragile X syndrome (FXS). A product of the study will be to understand the longitudinal trajectory of meaningful outcomes that impact the lives of children and adolescents with FXS with and without autism spectrum disorder (ASD), and from different racial/ethnic groups and geographic locations across the United States. The research will characterize cognitive, behavioral, and adaptive skills, and other indicators of functioning of individuals with FXS, and determine how these outcomes relate to diagnostic history, early intervention, and access to services. Finally, this study will collect data to explore the practical, financial, and psychological impact that caring for a family member with FXS has on caregivers and siblings of those with FXS. Post-award, outcomes to be measured will be chosen in coordination with CDC's Study to Explore Early Development (SEED) Follow-up Study (RFA- DD-21-001) and will include standardized in-person behavioral and functional assessments and follow-up computer-assisted telephone survey questionnaires. Data from this project and the SEED project will be collected and housed in a secure electronic data system maintained at a CDC-contracted organization. Outcomes chosen for study will facilitate comparison of health and functional outcomes for the FXS population with those for other related and frequently co - occurring conditions, such as ASD and developmental delay. Outcome data will assist with identifying public health gaps affecting those with FXS, inform service delivery recommendations and inform design of interventional studies, all with the goal of improving the lives of those with FXS and their families.