Type I is currently divided into seven subtypes, I-A to I-G; there are three distinct variants of the I-F subtype (I-F1, I-F2, and I-F3). A characteristic of Type I systems is CRISPR-associated complex for antiviral defense (Cascade), , which is a complex encompassing Cas molecules and crRNA. The Cascade proteins of Type I systems typically encompass Cas5, Cas6, Cas7, and Cas8 (Cse1), with some subtypes also presenting Cas4 and/or Cas11(Cse2). The pre-crRNA is processed by Cas6. In the interference stage of Type I systems, Cascade recognizes the PAM sequence, targets DNA in a sequence-specific manner through crRNA, and uses Cas3 to generates a single-strand nick on the unwinding target DNA thereby degrading the displaced strand through its 3′-5′ exonucleolytic activity. The overall composition and structure of Type III are highly similar to the Type I system. It utilizes multi-subunit effector complexes to cleave both RNA and DNA and has been classified into six subtypes: III-A to III-F. In the Type III system, the signature protein is multidomain Cas10 with an N-terminal histidine-aspartate (HD) nuclease domain (several subtypes) and two palm domains (a type of the RNA recognition motif). The HD domain takes part in nonspecific singlestranded DNA (ssDNA) cleavage activity, whereas the palm domain catalyzes the conversion of ATP to cyclic oligonucleotides (cOAs) when the type III crRNA-guided effector complex recognizes the target RNA. The cOAs activate the Csm6 protein, which nonspecifically degrades RNA molecules. Type IV CRISPR/Cas system is divided into IV-A, IV-B, and IV-C subtypes, which mainly exist in plasmids however, their specific function is still poorly characterized. Type IV lack certain hallmark components of other CRISPR-Cas systems, including the highly conserved adaptation module and an effector nuclease. All Type IV systems contain Cas7 protein (Csf2) and Cas5 protein (Csf3), which are part of the multi-subunit complex. Type IV-A CRISPR/Cas carry a DinG helicase instead of a nuclease component. Type IV CRISPR/Cas system may have the potential to be used in clinical drug-resistance gene therapy. Class 2 CRISPR/Cas systems Unlike Class 1 CRISPR/Cas system, the effector module of Class 2 CRISPR/Cas system is only a single protein with multiple domains and functions. Class 2 CRISPR/Cas systems are further divided into following: Type II, Type V and Type VI. For details regarding differences and similarities between subtypes of Class 2 systems, please refer to Table .