The invention relates to an approach for introducing one or more desired insertions and/or deletions of known sizes into one or more predefined locations in a nucleic acid (e.g., in a cell or organism genome). They developed techniques to do this either in a sequential fashion or by inserting a discrete DNA fragment of defined size into the genome precisely in a predefined location or carrying out a discrete deletion of a defined size at a precise location. The technique is based on the observation that DNA single-stranded breaks are preferentially repaired through the HDR pathway, and this reduces the chances of indels (e.g., produced by NHEJ) in the present invention and thus is more efficient than prior art techniques. The invention also provides sequential insertion and/or deletions using single- or double-stranded DNA cutting.
