FLOW CYTOMETRY CORE: Abstract The primary objective of the Joslin Diabetes Research Center (DRC) Flow Cytometry Core is to provide state- of-the-art equipment and expertise for quantitative phenotyping of cells and for isolating pure populations of cells from complex tissues. These approaches enable diabetes and metabolism investigators to study the development and cellular physiology of pancreatic islet, muscle, and adipose cells, as well as the inflammatory cells that contribute to autoimmune destruction of pancreatic beta cells, and to elucidate mechanisms in the pathogenesis of diabetic retinopathy, nephropathy and atherosclerosis. Services of this Core enable characterization of cellular phenotypes and rapid isolation of well-defined, pure, populations of live cells. Cell sorting technology is continuously evolving and improving, enabling new approaches to address key questions in diabetes research. In addition to continually updating equipment to offer cutting edge cell sorting technology to its users, the Core provides outstanding expertise through consultations and educational activities. During the past cycle, the Core was heavily used by diabetes investigators in the Joslin research base and the broader Boston diabetes research community, and use of Core services led to high impact publications that advanced the field. To meet the evolving needs of the research base, the Core acquired a new state-of-the-art flow cytometer (Becton Dickinson LSRFortessa) and an Ellispot reader, both of which are now heavily used. In addition, Core personnel provided training and education to users and carried out developmental research to improve methods. In the next cycle in collaboration with the Molecular Phenotyping and Genotyping Core, the Flow Cytometry Core will offer CITE-seq, an approach in which staining and flow cytometric analysis and separation of cells is performed up-front of single cell RNA sequencing and combined with DNA-barcoded antibodies that provide highly quantitative data on cell surface protein expression. CITE-seq will vastly expand the ability to phenotype markers simultaneous with unbiased gene expression analysis in cells of relevance to diabetes and metabolic disease. In sum, the Flow Cytometry Core provides critical equipment and expertise to advance our understanding of the pathophysiology of diabetes and its complications, and ultimately develop novel treatments and a cure for the disease and its complications.