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We recently reported a truncating deletion in the NFKBIE gene, which encodes IκBϵ, a negative feedback regulator of NF-κB, in clinically aggressive chronic lymphocytic leukemia (CLL). Preliminary data indicate enrichment of NFKBIE aberrations in other lymphoid malignancies, hence we screened a large patient cohort (n=1460) diagnosed with different lymphoid neoplasms. While NFKBIE deletions were infrequent in follicular lymphoma, splenic marginal-zone lymphoma, and T-cell acute lymphoblastic leukemia (<2%), slightly higher frequencies were seen in diffuse large B-cell lymphoma, mantle cell lymphoma, and primary CNS lymphoma (3-4%). In contrast, a remarkably high frequency of NFKBIE aberrations (46/203 cases, 22.7%) was observed in primary mediastinal B-cell lymphoma (PMBL) and Hodgkin lymphoma (3/11 cases, 27.3%). NFKBIE-deleted PMBL patients were more often therapy-refractory (P=.022) and displayed inferior outcome compared to wildtype patients (5-year survival: 59% vs. 78%; P=.034); however they appeared to benefit from radiotherapy (P=.022) and rituximab-containing regimens (P=.074). NFKBIE aberrations remained an independent factor in multivariate analysis (P=.003), also when restricting to immunochemotherapy-treated patients (P=.008). Whole-exome sequencing and gene expression-profiling verified the importance of NF-κB deregulation in PMBL. In summary, we identify NFKBIE aberrations as a common genetic event across B-cell malignancies and highlight NFKBIE deletions as a novel poor-prognostic marker in PMBL. [ABSTRACT FROM AUTHOR]