SUMMARY Over the last decade noncoding RNAs (ncRNAs) have emerged as prominent players that are involved in the regulation of most, if not all, biological processes. Given their important roles, it is conceivable that ncRNAs contribute to the development of diseases such as cancer. Circular RNAs (circRNAs) are ncRNAs that are formed through alternative splicing of linear pre-mRNAs via a process termed backsplicing. circRNAs are abundant, stable, and evolutionarily conserved and display cell-type specific expression patterns, indicating that they may play important roles in cell homeostasis, differentiation, development, and disease. However, the formation of circRNAs, their regulation, and their functions are poorly understood, partially due to the lack of tools to drive permanent ectopic expression of circRNAs. We have developed genetic tools for stable overexpression of candidate circRNAs in vitro and in vivo, which we will implement as part of this R21 proposal. Using these tools, we will determine the oncogenic potential of two circRNAs we hypothesize to promote melanomagenesis. We identified circPMS1 and circDNAJC2 as candidate melanoma oncogenes by a comprehensive circRNA expression analysis in benign melanocytes and malignant melanoma cells. We will test if overexpression of these circDNAs promotes melanocyte transformation in vitro and tumor formation in our high-throughput melanoma mouse models. We will also begin the molecular characterization of the functions underlying circPMS1 and circDNAJC2 oncogenic activity, with an emphasis on peptide translation and miRNA sponging. Moreover, we will specifically silence circPMS1 and circDNAJC2 in human melanoma cells in vitro as well as in xenograft models to examine if circRNA expression is required for maintaining the transformed state and continued melanoma growth, respectively. Our proposed studies will demonstrate that overexpression of circPMS1 or circDNAJC2 drives melanomagenesis and establish an easily adaptable experimental framework that will pave the way for future studies of oncogenic circRNAs.