Figure 1

ZFNs generate site-specific double-stranded breaks that can be used for homologous recombination or mutagenesis. (A) ZFNs are composed of two arrays that recognize 9-12 base pairs each. Two arrays with three fingers, F1-F2-F3, that recognize nine base pairs each are shown. Each array is fused to one half of a nonspecific Fok I endonuclease (green). Upon dimerization, the Fok I endonuclease is activated and creates a double-stranded break at sites flanked by the DNA binding sites recognized by the zinc finger arrays. Scissors and arrows denote the cut sites. (B) In most cells, double-stranded breaks (DSBs) are repaired by one of two major pathways. If a donor template is available, homologous recombination can result in engineered nucleotide substitutions at the target site (left). Alternatively, DSBs can be repaired by non-homologous end-joining, an error-prone mechanism that frequently results in small deletions or insertions at the site of the DSB (right).