As the fraction of cells with DSB is small and not all DSB are repaired by HR, we pursued an additional experimental strategy, using leukemia K562(HR-EGFP/3′EFP-MLL) and lymphoblastoid WTK1(HR-EGFP/3′EFP-MLL) cell lines (29) stably transfected with plasmid reporter comprising the highly fragile MLLbcr sequence (33). Here, KMT2A is linked to leukemia.