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© The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Main text Ewing sarcoma (EwS) is an aggressive boneor soft tissueassociated malignancy, characterised by the fusion oncoprotein EWSR1-FLI1 [1]. Over the past decades further therapeutic development for this devastating childhood tumour has remained relatively stagnant [2], especially for patients with metastatic or recurrent disease [3, 4]. To develop more effective and specific treatment options we investigated potential therapeutic targets by exploring putative downstream genes of EWSR1-FLI1. We took advantage of publicly available ‘omics’ data and filtered them in a multi-step approach (Fig. 1a): First, we interrogated a gene expression dataset comprising 50 primary EwS and 929 samples from 71 normal tissue types to identify overexpressed genes (min. log2 fold increase = 2) in EwS, which yielded 292 candidates (Fig. 1b, Supplementary Table 1). Second, we filtered for those genes whose overexpression was significantly negatively correlated with patients’ overall survival in a dataset of matched gene expression and survival data of 166 EwS patients [5] that covered 280 of the 292 overexpressed genes (96%) (Fig. 1c), identifying 22 candidates (Supplementary Table 1). Third, we focused on druggable targets possessing kinase or other enzymatic functions for which specific inhibitors and their pharmacokinetic data were available, but were still not (pre) clinically tested in EwS. This survey identified ribonucleotide reductase regulatory subunit M2 (RRM2) as the single putative target with a prominently negative association with patients’ overall survival (Fig. 1d). The ribonucleotide reductase (RNR) catalyses the conversion of ribonucleoside diphosphates to deoxyribonucleoside diphosphates, the rate-limiting process for de novo deoxyribonucleoside triphosphates synthesis. RNR is composed of two subunits, ribonucleotide reductase catalytic subunit M1 (RRM1) and either RRM2 or ribonucleotide reductase regulatory TP53 inducible subunit M2 (RRM2B) [6]. Notably, RRM2B is neither overexpressed in EwS nor negatively correlates with patients’ outcome (Supplementary Figs. 1a,b), and RRM1 is far less overexpressed in EwS compared to RRM2 (Supplementary Figs. 1a,b,c). Similar to primary EwS tumours, assessment of transcriptomes from 18 EwS cell line models (including A-673 and TC-71) also exhibited that, while RRM2 and RRM1 were similarly highly expressed in EwS Open Access