Project Summary Down Syndrome Regression Disorder (DSRD) is a serious neuropsychiatric syndrome that is of growing interest among the Down syndrome (DS) community and the INCLUDE Down Syndrome Research Plan. Adolescent and adult DSRD is associated with a subacute decline in cognitive and adaptive functioning, mood, and catatonia, with the highest risk period between 10 and 30 years old. Symptoms cause severe disability in the form of loss of ability to participate in activities of daily living and cause a great burden on parents and caregivers. Without prompt treatment, DSRD may persist for years. Recommended medical evaluation is costly and specialists are few and far between, making appropriate treatment difficult for some and inaccessible for others. There are also significant gaps in research on the prevalence and causes of DSRD, due in part to the lack of scalable assessments in large DS cohorts. Validated low-burden assessments are needed to identify patients, monitor treatment response, and improve research. This study aims to assemble a cohort of adolescents and young adults with DS and develop the DS- Regression Rating Scale (DSRRS), a parent-report form, which will aid in assessment and advance systematic research on the etiology and treatment of DSRD. To accomplish this, we will use a stakeholder- informed approach to assemble a cohort of 600 adolescents and young adults with DS, with and without DSRD. We will develop the DSRRS through an iterative process of focus groups, pre-testing, and item reduction. We will validate the newly created scale against well-validated assessments of symptoms associated with DSRD (e.g., stress, adaptive functioning, catatonia), examine the test-retest reliability, and examine sensitivity and specificity to optimize recommended cutoff scores. This project is in line with several priorities identified in the INCLUDE Down Syndrome Research Plan (pp. 13 – 14). These tools will lay the groundwork for vital research for years to come aimed at identifying the prevalence and etiologies, the gene-brain-behavior connections underlying DSRD, and most importantly, the identification of prevention and intervention strategies to improve the lives of people with DS at risk for DSRD.