Traditionally, targeted DNA modifications have required the use of plasmid-based donor repair templates that contain homology arms flanking the site of alteration54,55 (Fig. 2). The homology arms on each side can vary in length, but are typically longer than 500 bp (refs. 55,56). This method can be used to generate large modifications, including insertion of reporter genes such as fluorescent proteins or antibiotic resistance markers. The design and construction of targeting plasmids has been described elsewhere57.
