We propose to develop and use innovative in vivo and human islet cell systems to investigate the genetic basis of diabetes risk. Although dozens of loci and perhaps hundreds of positional-candidate genes, like the transcriptional regulators SIX3 and BCL11A, have been previously associated to increased diabetes risk by GWAS and other modern genome-scale findings, our understanding of the affected gene, impact on gene function, and impact on specific tissue(s) or cell-type(s) remains poor. Emerging data indicate that most of the common variant type 2 diabetes GWAS signals are driven by dysregulation of islet β-cells or α-cells. Current systems, including mice, have advanced our understanding of the properties of pancreatic β-cells and α-cells. However, diabetes research lacks in vivo systems to perform tissue-specific genetic loss- and gain-of-function studies on a scale or with the efficiency that could transform our understanding of diabetes risk genetics. Fortunately, based on our discoveries in the fruit fly Drosophila melanogaster, we have generated truly innovative in vivo genetic systems to discover with diabetes risk gene functions impacting insulin transcription, translation, processing, storage, secretion, and stability. Based on this workflow, we have identified several promising risk genes, including SIX3 and BCL11A, for complementary genetic studies in primary human islets. To investigate the function of prioritized human candidate diabetes-risk genes, we have created innovative methods permitting loss- and gain-of-function analysis in human primary β-cells. We have a strong scientific team with broad complementary expertise in human pancreatic islet biology and genetics, Drosophila genetics and physiology, pancreas and islet biology, developmental biology, and diabetes research. We have substantial experience in reliably procuring and using primary human islets for gene function studies. Here we propose to: (1) use powerful, innovative genetic and physiological assays in comprehensive in vivo studies to identify diabetes risk genes that regulate insulin biology and metabolism in Drosophila, and (2) use and develop quantitative assays for assessing cognate risk gene function in human islets. Together the proposed studies could transform understanding of functional diabetes genetics in human islets, and are therefore highly relevant to improving human health.