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When compared with class 1 systems, class 2 systems are much simpler in the sense that they comprise of a smaller number of components. Class 2 systems all have in common that the effector module is contained within a single protein, in contrast to multiple subunits of class 1 systems discussed so far. This class can be subcategorized based on the key genes involved in interference (Cas9, Cas12, and Cas13), all of which restrict the invading genomes in different ways. While this might not be optimal for a robust immune response, in particular when compared with type III systems, their simplicity makes them a better tool for genome editing purposes. While some class 1 systems have been used for gene editing, most notably in prokaryotes (Kiro et al. 2014; Li et al. 2016; Pyne et al. 2016), thanks to their simplicity (need for only one Cas protein) class 2 systems have been the easiest systems to be adapted for genome engineering. The many interesting activities of class 2 systems will be addressed below.