View source for Cas9 protein

Cas9 ([[Crispr|CRISPR]] associated [[Protein|protein]] 9) is a family of [[Endonuclease|endonucleases]] that could cut [[DNA|DNA]] at specific sites. The Cas9 proteins are shown to only function as a complex with two types of [[RNA|RNA]]:&nbsp;CRISPR-RNA ([[CrRNA|crRNA]]) and trans-activating CRISPR-RNA ([[TracrRNA|tracrRNA]])<ref>Jinek M, Chylinski K, Fonfara I, Hauer M, Doudna JA, Charpentier E. A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science. 2012 Aug 17;337(6096):816-21. doi: 10.1126/science.1225829</ref>. Both RNAs are required to guide the Cas9 proteins to specific cleaving sites on double stranded DNA.<br> 

In prokaryotes such as [[Streptococcus pyogenes|''S. pyogenes'']], the CRISPR-Cas9 system works as a defence mechanism to detect and cleave foreign DNAs that may come from [[Plasmid|plasmids]] or [[Bacteriophage|bacteriophages]]<ref>Jinek M, Chylinski K, Fonfara I, Hauer M, Doudna JA, Charpentier E. A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science. 2012 Aug 17;337(6096):816-21. doi: 10.1126/science.1225829</ref>. CRISPR-Cas9 can be applied in [[Genetic engineering|genetic engineering]]. Cas9 is shown to be useful in bacterial gene manipulation<ref>Jiang W, Bikard D, Cox D, Zhang F, Marraffini LA. RNA-guided editing of bacterial genomes using CRISPR-Cas systems. Nat Biotechnol. 2013 Mar;31(3):233-9. doi: 10.1038/nbt.2508</ref>. In human cells, Cas9 can be used to [[Gene knockout|knockout]] or edit [[Gene|genes]] with the use of custom guide RNAs (gRNA)<ref>Mali P, Yang L, Esvelt KM, Aach J, Guell M, DiCarlo JE, et al. RNA-guided human genome engineering via Cas9. Science. 2013 Feb 15;339(6121):823-6. doi: 10.1126/science.1232033</ref>. Cas9 proteins can be engineered to have higher levels of specificity when they target desired DNA sequences<ref>Slaymaker IM, Gao L, Zetsche B, Scott DA, Yan WX, Zhang F. Rationally engineered Cas9 nucleases with improved specificity. Science. 2016 Jan 1;351(6268):84-8. doi: 10.1126/science.aad5227</ref>.<br> 

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