Table 1.

Contrasting characteristics of the four standard nuclease platforms

Nuclease	Target site length	Mechanism of recognition	First use in human cells	Ease of design	Number of components	Size of mRNA transcript
Engineered meganuclease	>18 bp	Protein-DNA	1994 (I-SceI)	Extremely difficult	1	Short
Zinc-finger nuclease	18–36 bp	Protein-DNA	2003	Difficult	2	Short
TAL effector nuclease	24–40 bp	Protein-DNA	2011	Easy	2	Long
CRISPR/Cas9 nuclease	19–22 bp (Streptococcus pyogenes Cas9)	RNA-DNA Watson-Crick base-pairing	2013	Simple	1 (if using a complex guide RNA with Cas9 protein) or	Long
					2 (if delivering guide RNA and Cas9 separately)