@article{Makarova_Wolf_Iranzo-Evolu_class_CRISP-2020,
author = {Makarova, Kira S. and Wolf, Yuri I. and Iranzo, Jaime and 
          Shmakov, Sergey A. and Alkhnbashi, Omer S. and Brouns, Stan 
          J. J. and Charpentier, Emmanuelle and Cheng, David and Haft, 
          Daniel H. and Horvath, Philippe and Moineau, Sylvain and 
          Mojica, Francisco J. M. and Scott, David and Shah, Shiraz A. 
          and Siksnys, Virginijus and Terns, Michael P. and Venclovas, 
          Ceslovas and White, Malcolm F. and Yakunin, Alexander F. and 
          Yan, Winston and Zhang, Feng and Garrett, Roger A. and 
          Backofen, Rolf and van der Oost, John and Barrangou, 
          Rodolphe and Koonin, Eugene V.},
title = {Evolutionary classification of {CRISPR}-{Cas} systems: a 
         burst of class 2 and derived variants},
journal = {Nat Rev Microbiol},
year = {2020},
doi = {10.1038/s41579-019-0299-x},
volume = {18},
user = {alkhanbo},
pmid = {31857715},
pages = {67-83},
number = {2},
issn = {1740-1534},
abstract = {The number and diversity of known CRISPR-Cas systems have 
            substantially increased in recent years. Here, we provide an 
            updated evolutionary classification of CRISPR-Cas systems 
            and cas genes, with an emphasis on the major developments 
            that have occurred since the publication of the latest 
            classification, in 2015. The new classification includes 2 
            classes, 6 types and 33 subtypes, compared with 5 types and 
            16 subtypes in 2015. A key development is the ongoing 
            discovery of multiple, novel class 2 CRISPR-Cas systems, 
            which now include 3 types and 17 subtypes. A second major 
            novelty is the discovery of numerous derived CRISPR-Cas 
            variants, often associated with mobile genetic elements that 
            lack the nucleases required for interference. Some of these 
            variants are involved in RNA-guided transposition, whereas 
            others are predicted to perform functions distinct from 
            adaptive immunity that remain to be characterized 
            experimentally. The third highlight is the discovery of 
            numerous families of ancillary CRISPR-linked genes, often 
            implicated in signal transduction. Together, these findings 
            substantially clarify the functional diversity and 
            evolutionary history of CRISPR-Cas.}
}