ABSTRACT Ewing sarcoma is an aggressive malignancy that develops in bones or soft tissues of children and young adulthood. Although radiographic evidence of metastases at the time of diagnosis is associated with a particularly poor prognosis, approximately 50% of all patients will eventually relapse with disease refractory to conventional chemotherapy. Thus, there remains an urgent need to identify new biomarkers and therapeutic strategies to improve both prognosis and clinical outcomes, which requires a further understanding the molecular events driving Ewing sarcoma. The driving mutation in Ewing sarcoma is a chromosomal translocation that results in the fusion of the gene EWSR1 and one of a few ETS transcription factor, most commonly FLI1. Our lab, together with others, have shown that the aberrant chimeric EWS-FLI1 fusion transcription factor gains oncogenic activity by maintaining accessible chromatin at disease-specific regions leading to Ewing sarcoma development by regulating a number of genes. As the EWS-FLI1 fusion is exclusively present in the tumor cells, it has been considered as an ideal drug target; however, in the absence of targetable activities and binding domains, EWS-FLI1 continues to be a therapeutic challenge. A reductionist approach based on the identification and targeting of individual molecular targets has also largely failed. However, mechanisms that regulate EWS-FLI1 protein is just beginning to be appreciated. EWS-FLI1 is post-translationally modified by phosphorylation and glycosylation; however, regulation of EWS-FLI1 stability remains unstudied. Destabilizing EWS-FLI1 offers a new strategy to control EWS-FLI1 activity. Our preliminary studies have identified the E3 ubiquitin ligase SPOPCUL3 as the first E3 ubiquitin ligase to promote EWS-FLI1 degradation, and the deubiquitinase OTUD7A to antagonize the function of SPOPCUL3. In this proposal, we will investigate the molecular mechanisms and pathological functions underlying both SPOPCUL3 and OTUD7A mediated EWS-FLI1 stability control in Ewing sarcoma, as well as further investigate the feasibility of therapeutic targeting OTUD7A as a new approach to treat Ewing sarcoma patients with EWS-FLI1 fusion.