Table 1.

Comparison of ZFN, TALEN, and CRISPR/Cas

Technology	Working mechanism	Essential components	Efficiency	Criteria for target site	Refs.
ZFN	DNA/protein recognition	ZFN with zinc finger domain and FokI endonuclease domain	Variable	Preferential binding sequence for zinc finger proteins or Cys2His2 fingers	[2], [3], [4]
TALEN	DNA/protein recognition	TALE and FokI fusion protein	High but variable	TALE binding sites should start with a T with the space between two TALEN arms better 15–21 bp	[15], [16], [17], [18], [19]
CRISPR/Cas	DNA/RNA recognition	Cas9 protein and gRNA	High but variable	gRNA target site should be 20 bp long starting with a G for U6-directed transcription and GG for T7-directed transcription; PAM sequence (NGG) is indispensable for Cas9 nuclease activity	[28], [29], [30], [31]

Note: ZFN, zinc finger nuclease; TALEN, transcription activator-like effector nuclease; CRISPR, clustered regularly-interspaced short palindromic repeat; Cas, CRISPR-associated; gRNA, guide RNA; PAM, protospacer adjacent motif.