From: Current applications and future perspective of CRISPR/Cas9 gene editing in cancer
 | Type-I | Type-II | Type-III |
---|---|---|---|
CRISPR-cas action | |||
 Adaptation | |||
  Whether to depend on PAM when selecting proto-spacers | Yes | Yes | No |
 Expression | |||
  Pre-crRNA conjugates | Cascade complex | Cas9 (Csn1/Csx12) tracrRNA | Cas6 Csm (subtype III-A)/ Cmr (subtype III-B)a |
  Pre-crRNA cleavage enzymes | Cas6e subunit (subtype I-E)/Cas6f subunit (subtype I-F) | Housekeeping RNase III | Cas6 |
  Processes to mature crRNA | 1. A typical 8-nucleotide repeat fragment on the 5′ end 2. A hairpin structure on the 3′ flank | Cleavage at a fixed distance within the spacers (probably catalyzed by Cas9) | 1. Cas6 is responsible for the processing step 2. Trimming the 3′ end of the crRNA further (Nucleases have not yet been identified.) |
 Interference | |||
  Methods of target recognition | Cascade complex guided by crRNA | Cas9 loaded with crRNA directly | The invading DNA fragment having no base pairing to the 5′ repeat fragment of the mature crRNA (resulting in interference) |
  Targets cleavage enzymes | Cas3 | Cas9 | Cas6 or Cmr/Csm complex |
  Targets | DNA | DNA | DNA (III-A)/ RNA (III-B) |
  Whether to depend on PAM to cleaving process | Yes | Yes | No |
Special systems contained | cas3 gene | ‘HNH’-type system | Polymerase and RAMP modules |
Distribution of the three types of CRISPR-Cas systems | More common in Archaea | Only in Bacteria | More common in Archaea |