Despite extensive characterization of the role of the EWS-ETS fusions, little is known about secondary genetic alterations and their clinical contribution to Ewing sarcoma (ES). It has been demonstrated that the molecular structure of EWS-ETS lacks prognostic value. Moreover,CDKN2Adeletion andTP53mutation, despite carrying a poor prognosis, are infrequent. In this scenario identifying secondary genetic alterations with a significant prevalence could contribute to understand the molecular mechanisms underlying the most aggressive forms of ES. We screened a 67 ES tumor set for copy number alterations by array comparative genomic hybridization. 1q gain (1qG), detected in 31% of tumor samples, was found markedly associated with relapse and poor overall and disease-free survival and demonstrated a prognostic value independent of classical clinical parameters. Reanalysis of an expression dataset belonging to an independent tumor set (n= 37) not only validated this finding but also led us to identify a transcriptomic profile of severe cell cycle deregulation in lqG ES tumors. Consistently, a higher proliferation rate was detected in this tumor subset byKi-67immunohistochemistry.CDT2,a 1q-located candidate gene encoding a protein involved in ubiquitin ligase activity and significantly overexpressed in 1qG ES tumors, was validatedin vitroandin vivoproving its major contribution to this molecular and clinical phenotype. This integrative genomic study of 105 ES tumors in overall renders the potential value of 1qG andCDT2overexpression as prognostic biomarkers and also affords a rationale for the application of already available new therapeutic compounds selectively targeting the protein-ubiquitin machinery.