KMT2A and leukemia: The researchers then used a human lymphoblast cell line derived from patients with chronic myeloid leukemia (CML) and a custom-designed ZFN to deliver DSBs to specific sites of the telomeric region of the mixed-lineage leukemia (MLL) gene breakpoint cluster region, as well as to analyze chromosomal structural abnormalities in MLL leukemogenesis by repairing DSB errors [8,9,10].