PROJECT SUMMARY/ABSTRACT Autistic females (iASD) and females with fragile X syndrome (FXS) are two clinical groups that are significantly understudied in research. Despite the fact that, like their male peers, females experience pervasive, lifelong difficulties with language, adaptive functioning, psychiatric comorbidities, and academic achievement, there remains a dearth of research regarding phenotypic development in iASD females and females with FXS. FXS is the most common inherited cause of intellectual disability and the most common single gene cause of ASD, with 14-45% of females with FXS having a co-occurring ASD diagnosis. Research on males has identified key areas of overlap and distinction between FXS and iASD, particularly in language and cognition. Thus, studies comparing the phenotypes of FXS and iASD in females, focusing on areas of overlap and divergence, are particularly important because they provide the foundation for clinical and educational planning and elucidating the mechanisms producing the phenotypes of FXS and iASD. Executive functions are one aspect of cognition that has received significant attention in FXS and iASD, since they are an important contributor to real-world outcomes and particularly amenable to treatment. The hierarchical competing systems model of executive functions describes the interrelationships between language and executive functions in neurotypical development, but these relationships have yet to be explored in iASD females and females with FXS. Both language and executive functions are known to have a significant and pervasive impact on critical developmental outcomes including academic skills (i.e., literacy), adaptive behavior, and psychiatric symptomatology. The impact of language and executive functions on these outcomes has not been explored in iADS females and females with FXS. There is an urgent need to characterize the language and cognitive phenotypes of school-age iASD females and females with FXS within a developmental framework to understand the nature and course of development, and impact over time on important outcomes. Through three specific aims, we will determine the extent, nature and syndrome specificity of impairments in language and executive functions in iASD females and females with FXS, and test their causal relationships through longitudinal associations. We will recruit 8-12 year-old females with FXS (n = 60) and iASD females (n = 60). The proposed study will include standardized assessments, expressive language samples, experimental tasks, and parent report measures taken at two time points. We will determine the impact of language and executive functions on key developmental outcomes including literacy, adaptive functioning, and psychiatric symptomatology. The data from this study will provide critical information for clinicians and educators working with iASD females and females with FXS and will provide data necessary for the development of targeted interventions for c...