Furthermore, DOT1L and H3K79 methylation were found to be essential for ionizing radiation (IR)-induced 53BP1 foci formation during G1/G2 phase, and DOT1L depletion in cancer cells led to prolonged presence of phosphorylated histone H2AX on Ser139 (γH2AX) post-IR, suggestive of defective DNA repair [18]. The gene discussed is DOT1L; the disease is cancer.