Fig. 1: Classification of the CRISPR system and the operon organization for each subtype.

Genes belong to any step composed of the chromosomal integration of invaded DNA (adaptation), the production of CRISPR RNA (expression), and the cleavage of exogenous DNA (interference). The class 2 CRISPR system is characterized as a single nuclease effector protein, and type II, V, and VI constitute the Class 2 system. Types II and V show DNA cleavage activity, while type VI uses ssRNA as a substrate. A systematic name is given for all effector nucleases with a legacy name in parentheses for several nucleases. The dashed arrows indicate that cas1 and cas2 genes are present in only some bacterial or archeal chromosomes. The X mark denoted in Cas12k refers to a catalytically inactive RuvC-like domain