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Recurrent driver mutations in genes encoding histone H3 (H3.3K27M and H3.1K27M) are observed in ~80% of diffuse midline gliomas (DMG), which lead to aberrant gene regulation, yet the specific RNA polymerase II (Pol2) regulators that induce transcriptional dysregulation in DMG are not fully defined. We identified multiple regulators of Pol2 elongation as DMG genetic dependencies in a chromatin-focused CRISPR screen. Additional studies confirm that knockout (KO) of the Pol2 SIII complex gene elongin B (ELOB) inhibits DMG cell proliferation in tissue culture and tumor growth in xenograft models. Further genomic analyses reveal that ELOB binding sites are enriched in H3K27M oncohistones and that ELOB KO alters H3K27me3 and H3K27M incorporation at thousands of genomic regions, implicating ELOB in the maintenance of dysfunctional chromatin states in DMG. Correspondingly, PRO-seq and RNA-seq profiling reveal that ELOB loss disrupts Pol2 transcriptional activity and alters the expression of transcripts involved in metabolism, proliferation, and brain development. These findings suggest that Pol2 elongation factors like ELOB cooperate with H3K27M oncohistones to maintain the epigenetic and transcriptional landscape driving DMG malignancy.