CHD4 is necessary for the maintenance of heterochromatin and the efficient repair of DNA DSBs in AML blasts. AML cell lines were infected by lentivirus to integrate either a nontargeting, scrambled (Sc) shRNA or a CHD4-targeting shRNA. (A) Depletion of CHD4 leads to a global increase in euchromatin-associated histone H3K9 and H4K8 acetylation. (B) Depletion of CHD4 also results in a disruption of heterochromatin-associated histone H3K9me3 foci in U937 cells as shown by staining with anti-H3K9me3 antibody. (C) U937 cells were exposed to 6 Gy of radiation to induce DNA DSBs. DSB formation and repair was monitored over a 4-hour time course using a neutral comet assay. CHD4-depleted cells displayed significantly more evidence of DSBs and were delayed in their repair (n > 50 comets). (D) The results of the comet assay were confirmed by staining for Υ-H2A.X foci and calculating the fluorescent intensity per nucleus (n > 60 cells per time point). (E) Representative images of the Υ-H2A.X stain. *P < .05.