Table 1.

Comparison of the Cas9 and Cas13 systems.

Cas Effectors	Cas9	Cas13
Protein size	~1100 aa	~775–1250 aa
Targeted substrate	dsDNA	RNA
PAM/PFS	The PAM region of spCas9 was NGG, while that of saCas9 was NNGRRT	Different Cas13 family types have different PFS requirements, among which Cas13a tends to A, U, and C, Cas13b tends to A, U, and G, while Cas13d and Cas13X have not been found to have PFS.
Targeted site distribution	Due to the limitation of the PAM region of Cas9, the distribution of editable sites is relatively general.	Due to the limitation of the PAM region of Cas13a and Cas13b, the distribution of editable sites is relatively general. However, for Cas13d and Cas13X, no obvious PFS region has been found, which has an extensive selection of editing sites.
Trans-cleavage	No trans-cleavage activity was found	Cas13a, Cas13b, and Cas13X were found to have significant trans-cleavage activity.
Damage to organisms	There’s permanent damage to the genome	Because the Cas13 family effectors target editing at the RNA level, they rarely cause permanent genetic damage to cells or organisms.
Application	Knockin and knockout of genes; single base mutation in DNA; epigenetic regulation	Basic biochemical research; nucleic acid detection and diagnosis; nucleic acid imaging technology; antiviral applications; disease treatment strategies based on CRISPR/Cas13