Table 1.

Summary of engineered Cas9 variants for gene editing applications.

Cas9 Variants	Advantages	Disadvantages
Wild type Streptococcus pyogenes Cas9 (spCas9)	Programmed RNA guided editing; High specificity; Easily engineered	dsDNA breaks repaired by NHEJ forming indels
Cas9 nickase (Cas9n)	No dsDNA break induced; Promotes homology directed repair (HDR)	Some nicks go through a dsDNA break intermediate that can be repaired by NHEJ
Dual sg-RNA-Cas9 nickases (Cas9dn)	Increased specificity, dual sgRNA, promotes higher HDR over single nickase.	Must design dual sg-RNA-Cas9n complexes targeting opposite DNA strands
Cytidine deaminase fused Cas9 (D10A)	No dsDNA break induced; Increased efficiency over spCas9; Direct base conversion of C→T	Five base pair editing window; specific C→T conversion
spCas9-Gem	Regulates Cas9 presence at each stage of the cell cycle; Efficiently generates knock-in reporter lines and gene correction	Decreases frequency of NHEJ indels at target locus

Abbreviations: DSB = double-stranded DNA break; NHEJ = Non-homologous end joining; HDR = Homology directed repair.