MicroRNAs (miRNAs) have recently emerged as a major class of trans-factors that regulate expression of protein coding genes through their 3'UTRs, thereby controlling a diverse range of biological processes including cell differentiation, proliferation, and apoptosis. miRNAs pair with mRNAs of their target genes and the usual consequence is the downregulation of protein expression by translational repression, mRNA cleavage, or promotion of mRNA decay. Hundreds of miRNAs, many of them evolutionarily conserved, have been identified in mammals, and a fraction of these molecules exhibit highly specific, regulated expression patterns in the immune system. Genetic studies from us and other groups have demonstrated that miRNAs play critical roles in lymphocyte development, immune responses, and lymphomagenesis. However, little is known about the roles of miRNAs in autoimmune diseases. A large amount of genetic and epigenetic alterations in miRNA coding genes have been found in human patients. Among them is the amplification of the miR-17~92 gene, which encodes six distinct miRNAs, and the elevated expression of miR-17~92 miRNAs in cells carrying this gene amplification. We have generated a miR-17~92 transgene whose expression can be turned on conditionally by Cre recombinase in mice. Strikingly, the transgenic mice developed a lymphoproliferative and autoimmune disease, and died prematurely, when this transgene was turned on in both B and T lymphocytes using hCD2-iCre. Our preliminary studies showed that transgenic miR-17 ~ 92 expressions broke B cell tolerance in an anti-IgM macroself (5b-ms) superantigen transgenic model. We hypothesize that transgenic miR-17~92 expression causes autoimmune diseases mainly by breaking B cell tolerance. We now propose studies to elucidate the cellular and molecular mechanisms underlying miR-17~92 mediated breaking of B cell tolerance and development of autoimmune diseases, and to illustrate how alterations in miRNA expression contribute to autoimmunity and how miRNA clusters carry out their functions. We will determine the contribution of transgenic miR-17~92 expression in B cells to the autoimmune disease (Aim 1), assess the impact of transgenic miR-17~92 expression on B cell tolerance checkpoints (Aim 2), dissect the functional contribution of individual miRNAs in the miR-17~92 cluster to the breaking of B cell tolerance (Aim 3), and identify target genes and molecular pathways whose deregulation leads to the autoimmune disease in miR-17~92 transgenic mice (Aim 4). We will combine genetic, proteomic, genomic, molecular, and bioinformatic approaches to achieve these goals.