Table 3.

Main differences between HEs, ZFNs, TALENs, and CRISPR-Cas genome editing systems.

Features	HEs	ZFNs	TALENs	CRISPR-Cas
Origin	Mobile genetic elements in microbiome	Eukaryotic transcription factors	TALENs of plant pathogenic bacteria	Bacterial adaptive immune system
Target sequence (bp)	20–30	9–18	14–20	~23
Number of target sites	Limited	Various	Various	Various
Engineering	Simple	Difficult	Slightly difficult	Very simple
Size (kb)	~1	~1	~3	>3
Target recognition	Protein-DNA	Protein-DNA	Protein-DNA	RNA-DNA and Protein-DNA
Specificity/ Off-target effects	High/Low	Low/High	High/Low	Highest/Low to high
Targeting efficiency	Low to variable	Variable	Variable to high	High
Multiple targeting	No/Protein engineering for each new target	No/Protein engineering for each new target	No/Protein engineering for each new target	Yes/Multiple targets can be edited simultaneously