PROJECT ABSTRACT Despite continued calls for increased diversity in research samples of individuals with Down syndrome (DS) and other neurodevelopmental disorders, researcher success in obtaining these samples is limited. Reasons for low enrollment of diverse participants include historic scientific injustices, mistrust, lack of culturally sensitive information and awareness about the research process, and time and resources constraints. The proposed supplement will begin to address these barriers by engaging in activities to build community relationships and establish trust, with the goal of obtaining a more ethnically, racially, and geographically diverse sample. First, we will add a recruitment coordinator who is a member of one of the underrepresented communities from which we will recruit. Then, the recruitment coordinator will (a) assemble a community advisory panel comprised of stakeholders (e.g., parents, professionals, and self-advocates from targeted communities) to provide advice and feedback about recruitment strategies, and b) create and distribute culturally tailored recruitment materials for more diverse audiences (as identified by the panel). The overarching goal of the proposed supplement is to enroll at least 150 individuals with DS from ethnically, racially, and geographically diverse backgrounds into the parent project (total sample size = 500). The purpose of the parent project to which the proposed supplement will contribute is to examine the reliability, validity, and variability of three well-known autism spectrum disorder (ASD) symptom measures in a large, diverse, national sample of 6- to 18-year-olds with DS. We will leverage data from these ASD measures, along with additional deep phenotyping, to characterize the heterogeneity of the co-occurring ASD phenotype in DS and identify symptom profiles. Additionally, an exploratory aim among a subsample (n = 25) at high or low ASD risk will examine the feasibility of tele-assessment methods for gathering direct, performance-based ASD evaluations. Data generated from this project will enhance clinical trial readiness by providing ASD measures in DS that can (a) screen for ASD risk to identify candidates for treatment, (b) stratify cohorts by ASD symptom profiles, and (c) monitor response to treatment across these profiles. The exploratory feasibility study will determine the extent to which tele-assessments can be used for performance-based ASD evaluations in children with DS. The knowledge gained will prepare the field for conducting clinical trials remotely, which will improve access to care across geographically, racially, and ethnically diverse communities. Together, the parent project and supplement address multiple NIH INCLUDE and NICHD IDD Branch priorities, especially (a) increase the likelihood of clinical trial success through testing of clinical outcome assessment measures, (b) define the presentation and course of co-occurring conditions in individuals with DS, an...