PROJECT SUMMARY/ABSTRACT Down syndrome is the most prevalent genetic condition in humans and a major cause of intellectual disability. Although the severity and extent of phenotypes present in Down syndrome partially result from an extra copy of chromosome 21, details regarding the biological mechanisms and the emergence of atypical development is not understood. To address this significant gap in knowledge, we previously created a ‘Developmental Cell Atlas of Down Syndrome’ by using single-cell RNA-sequencing to profile the transcriptomes of over 700,000 cells derived from multiple tissues. This proposal represents our ongoing efforts to characterize the molecular and cellular identity of cellular phenotypes present in the developing brain in Down syndrome. In Aim 1, we will map the Down syndrome cells onto a reference framework of the developing human brain by pooling existing single-cell RNA-sequencing data. In Aim 2, we will use our established human brain functional genomics pipeline to infer cell-type-specific gene regulatory networks altered in Down syndrome. Leveraging existing data, this study will provide critical information about the emergence and regulation of early brain development in Down syndrome that can be used to elucidate the molecular mechanisms underlying early neuropathology and to benchmark model systems for disease relevant neuronal phenotypes.