Table 1.

Characteristics of CRISPR-Cas systems

Class	Type/Subtype	Effector	Nuclease domains	Target	Cut structure	tracrRNA requirement	PAM/PFS	Application
1	I	Cascade	HD nuclease domain of Cas3	DNA	Single-strand cut (200-300nt)	NO	PAM	Genome editing, antimicrobials, gene regulation in bacteria and archaea
	III	Csm/Cmr complex	Cas10 PALM domain Cas7 Csm/Cmr complex	DNA RNA	Multiple sites	NO	Independent of PAM	Genome engineering and gene silencing
	IV	Complex	HD nuclease domain	DNA	Double strand	NO	PAM	Controlling plasmid propagation
2	II	Cas9	RuvC, HNH	dsDNA RNA	Blunt	Yes	3′ GC-rich PAM	Elimination of repetitive sequences specific Gene editing RNA knockdown RNA isolation (dCas 9) RNA imaging and tracking (dCas 9) Resistance against RNA viruses (Fn Cas9) Regulation of gene expression
	V-A	Cas12a (Cpf1)	RuvC, NUC	dsDNA	Staggered, 5′-overhangs(7nt)	No	5′ AT-rich PAM	Gene editing Nucleic acid detection
	V-B	Cas12b (C2c1)	RuvC	dsDNA	Staggered, 5′overhangs (5nt)	Yes	5′ AT-rich PAM	Nucleic acid detection
	VI-A	Cas13a (C2c2)	2xHEPN domain	ssRNA	Guide-dependent RNA cuts + collateral RNA cleavage	No	3′ PFS: non-G	RNA knockdown RNA imaging and tracking (dCas13a) Nucleic acid detection Resistance against RNA viruses
	VI-B	Cas13b (C2c6)	2xHEPN Domain	ssRNA	Guide-dependent RNA cuts + collateral RNA cleavage	No	5′ PFS: non-C 3′ PFS: NANA/NNA	RNA knockdown RNA editing Regulation of gene expression Nucleic acid detection

dsDNA, double-stranded DNA; ssRNA, single-stranded RNA; PAM, protospacer adjacent motif; PFS, protospacer-flanking sequence