Figure 1.

Schematic overview of the three main gene editing tools. A, Zinc finger nuclease (ZFN), consisting of a DNA‐cutting nuclease domain (gray box), and a protein‐based DNA‐binding domain of three zinc finger proteins (colored circles), each recognizing a three base pairs (bp) DNA sequence. Hence, this ZFN recognizes a 9 bp genomic sequence. B, Transcription‐activator like effector nuclease (TALEN), consisting of a DNA‐cutting nuclease domain (gray box), and a protein‐based DNA‐binding domain of 18 TAL effector repeats. Each TAL effector consists of 34 amino acids, typically highly conserved, with positions 12 and 13 being variable and determining the specific recognition of one DNA bp. Hence, this TALEN recognizes a 18 bp genomic sequence. C, Clustered regularly interspaced short palindromic repeats (CRISPR)‐Cas9 system, consisting of a DNA‐cutting nuclease (gray box), with two sites of nuclease activity, and a RNA‐based DNA‐binding domain consisting of a single guide RNA (sgRNA), with the variable 20 nucleotide RNA‐sequence determining recognition of a 20 bp complementary genomic sequence