11532 Background: Ewing Sarcoma (ES) is an aggressive, translocation-associated, bone cancer associated with a poor prognosis in the recurrent or metastatic setting. ES is identified by the canonical balanced reciprocal chromosomal translocation involving EWSR1 and ETS transcription factors. Secondary somatic alterations in ES are rarely described and genomic alterations (GA) affecting various molecular pathways may work synergistically with ETS-FL1 translocations to promote oncogenesis. Alterations in fibroblast growth factor receptor 4 (FGFR4), a receptor tyrosine kinase protein that functions in cellular processes, have been observed to affect carcinogenesis. Moreover, the FGFR4-Gly388Arg (G388R) single nucleotide polymorphism (SNP) is found to increase the risk of cancer with mouse embryonic fibroblasts derived from knock-in strain of homologous Fgfr4 G385R mice exhibiting a transformed phenotype. We sought to further evaluate the frequency of FGFR4 G388R SNP in relation to other identifiable pathway alterations observed with Comprehensive Genomic Profiling (CGP). Methods: Next generation sequencing (NGS) analysis was obtained in the context of clinical care with clinical status, outcomes, and source acquisition (primary tumor, metastasis, or recurrence) unknown to Foundation Medicine. CGP, FoundationOne Heme, evaluated GAs including base substitutions, indels, amplifications, copy number alterations, gene fusions and rearrangements. 189 samples were assayed by hybrid-capture based CGP, including 406 DNA-sequenced genes in addition to 265 RNA-sequenced genes commonly reported to be rearranged in cancer, as previously described. Tumor mutational burden was assessed from a minimum 1.4 Mb sequenced DNA. Microsatellite instability (MSI) status was determined by a novel algorithm analyzing 114 specific loci. Results: The median age of evaluated patients was 20 (range 0-70) with the number of alterations averaging 7 per patient. Pathways noted to be altered in the presence of FGFR388R SNP occurred frequently with the following pathways most observed: MAPK (33%), WNT (32%), NOTCH1 (20%), HRR (19%), Histone/chromatin remodeling (18%). FGFR388R SNP was observed in more than half (51%) of evaluated samples. Most affected pathways irrespective of FGFR388R SNP status included: MAPK (n = 89), HRR (n = 75), and PIK3 (n = 64). All evaluated samples were TMB low ( < 10 mut/mb) and Microsatellite Stable. Conclusions: Secondary GAs affecting major pathways were observed in high frequency, often co-occurring with the FGFR4 G388R SNP. Secondary alteration of known oncogenic pathways may contribute to sarcoma formation in ES potentially informing further therapeutic strategies in the future.