Fig. 2

DSB-mediated genome editing. Upper left: type of site-specific endonucleases which are recently used for efficient genome editing purposes. Upper right: introduction of a null mutation by DSB. When repaired by NHEJ pathway, small deletion or insertion of nucleotides (indels) occurred at the joint site, which cause a nonsense or missense mutation in the targeted ORF. Long deletions can also be introduced by multiple DSBs. Lower panels: strategies of fragment insertion. Homology-directed repair (HDR) supports insertion of a large or a small fragment with homology sequences. NHEJ also supports the insertion of a large fragment without homology sequence, although inserted direction is not controllable and indels are introduced at the joint regions. Microhomology-mediated end joining (MMEJ) mediates fragment insertion with very short (10–40 bp) microhomology arms and thus potentially ameliorates drawbacks in the other two pathways