Table 1.
Key features of gene editing tools.
| ZFN | TALEN | CRISPR | |
|---|---|---|---|
| Target sequence | 9–18 bp per ZFN monomer | 14–20 bp per TALEN monomer |
20 bp guide sequence plus PAM sequence |
| Recognition site | Zinc-finger protein | TALE protein RVD tandem repeat region of | Single-stranded single guide RNA (sgRNA) |
| Mode of recognition | Protein:DNA ZF modules; interference of neighboring recognition modules | Protein:DNA TALE RVDs; clear 2:1 amino acid:nucleotide code | RNA:DNA; 1:1 Watson–Crick base pairing |
| Endonuclease | FokI | FokI | Cas |
| Size (kb) | ~1 | ~3 | ~3.5–4.5 |
| Ease of engineering | Complicated—Requirement of substantial protein engineering | Simplified—Requirement of complex molecular cloning procedures | Simplest—Use of 20 nucleotide sgRNA sequence per target site |
| Off-target effects | High | Low | Variable |
| Size (kb) | ~1 | ~3 | ~3.5–4.5 |
| Ease of engineering | Complicated—Requirement of substantial protein engineering | Simplified—Requirement of complex molecular cloning procedures | Simplest—Use of 20 nucleotide sgRNA sequence per target site |
| Off-target effects | High | Low | Variable |
| Cost | High | Moderate | Low |
Abbreviations: ZFN—Zing finger nucleases; TALEN—Transcription activator-like effector nucleases; CRISPR—Clustered regularly interspaced short palindromic repeats.