The RNA guided enzyme Cas9, which originates from the CRISPR-Cas adaptive bacterial immune system, is transforming biology by providing a genome engineering tool based on the principles of Watson-Crick base pairing. The CRISPR associated protein Cas9 is an endonuclease that uses a guide sequence within an RNA duplex, tracrRNA:crRNA, to form base pairs with DNA target sequences, enabling Cas9 to introduce a site-specific double-strand break in the DNA. Bioinformatic analyses first identified Cas9 (formerly COG3513, Csx12, Cas5, or Csn1) as a large multifunctional protein with two putative nuclease domains, HNH and RuvC-like. The Cas9 protein undergoes large conformational rearrangement upon binding to the guide RNA, with a further change upon association with a target doublestranded DNA (dsDNA). 
This change creates a channel, running between the two structural lobes of the protein, that binds to the RNA-DNA hybrid as well as to the coaxially stacked dualRNA structure of the guide corresponding to the crRNA repeat–tracrRNA antirepeat interaction. The CRISPR-Cas9 technology originates from type II CRISPR-Cas systems, which provide bacteria with adaptive immunity to viruses and plasmids.  The technology can also facilitate the generation of mouse and rat models better suited to pharmacological studies and the understanding of human diseases, as well as pigs and monkeys. Overall, CRISPR-Cas9 is already having a major impact on functional genomic experiments that can be conducted in these model systems.


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