Table 2.

Characteristics of three genome editing systems

Nucleases	ZFN	TALEN	CRISPR/Cas
DNA binding domain	Multiple zinc finger peptides	Transcription-activator like effectors	CRISPR-derived RNA/Single-guide RNA
Endonuclease	Fok1	Fok1	Cas9
Binding specificity of each repeat	3 bp	2 bp	1 bp
Target site length	18 to 36 bp	30 to 40 bp	23 bp
Off-target	High probability	Low probability	Variable
Libraries generation	No	Feasible, depend on technology	Yes, cloning 20 bp, oligos targeting each gene into a plasmid