Table 1.

Overview of CRISPR–Cas classification and characteristics [17,18,19,20].

Class	Type	Effector Module	Nuclease Domain	RracrRNA	Cleaving Target
I	I	Multiple Cas proteins—Cas3 (sometimes fused to Cas2), Cas5–Cas8, Cas10 and Cas11 in different combinations, depending on the type and subtype	Histidine-aspartate domain (HD) fused to Cas3	No	DNA
	III		HD fused to Cas10	No
	IV		Unknown	No
II	II	Cas9 protein	RuvC and HNH	Yes
	V	Cas12a (Cpf1)/Cas12b/Cas12c	RuvC and Nuc	Cpf1-No
	VI	Cas13a/Cas13b/Cas13c	Higher eukaryotes and prokaryotes nucleotide-binding (HEPN) domains	No	RNA