Table 1.

Features of CRISPR-Cas systems by type.

Class	Features	Type	Features	Key Effectors	Target	Ref.
1	Effector, adaptation, and accessory functions distributed over multiple proteins	I	Assembly of multiple Cas proteins into the signature CRISPR-associated complex for antiviral defense (Cascade)	Cas1, Cas2, Cas4, Cas5, Cas6, Cas3, Cas8	dsDNA	[25,49,50]
		III	Signature multimeric complex known as Csm/Cmr Some effectors in this type use spacers produced by Type I systems	Cas1, Cas2, Cas5, Cas6, Cas7, Cas10	dsDNA, RNA	[51,52]
		IV	Often lacks adaptation module genes (Cas1 and Cas2) Involved in competition between plasmids in bacteria	Cas5, Cas7, Csf1	dsDNA	[52,53]
2	Single protein with multiple domains combines crRNA-binding, catalytic activity, and pre-crRNA processing	II	Mainly DNA binding Contains two metal-dependent nuclease domains (HNH and RuvC) Requires PAM Relies on RNase III to process its crRNA	Cas9	dsDNA, RNA	[12,25,54,55]
		V	Requires PAM Reduced off-target activity compared to Cas9 Collateral RNA or ssDNA cleavage in some subtypes	Cas12, Cas14	dsDNA, ssDNA, RNA	[25,56]
		VI	Two HEPN nuclease domains collateral RNA cleavage Some orthologs inactive in mammalian cells No PFS dependency in some subtypes Processes its own crRNA	Cas13	RNA	[15,25,41,54]