text: Developing a new sgRNA library is a laborious and time-consuming process. In practice, researchers may select an existing library depending on their experimental purpose and cell lines of interest.
  As of February 2020, the most widely used resources for genome-wide CRISPR knockout screens have been the two [https://www.addgene.org/pooled-library/zhang-human-gecko-v2/?gclid=Cj0KCQiA1-3yBRCmARIsAN7B4H3q-L7oXzYqaazZYhsLG6hugYsIOXTQrBe2cJMQywz37qDGmWcuhvwaAmfkEALw_wcB/ Genome-Scale CRISPR Knock-Out (GeCKO)] libraries created by the Zhang lab. Available through Addgene, these lentiviral libraries respectively target human and mouse exons, and both are available as a one-vector system (where the sgRNAs and Cas9 are present on the same plasmid) or as a two-vector system (where the sgRNAs and Cas9 are present on separate plasmids). Each library is delivered as two half-libraries, allowing researchers to screen with 3 or 6 sgRNAs/gene.
  Aside from GeCKO, a number of other CRISPR libraries have been generated and made available through Addgene. The Sabatini & Lander labs currently have 7 separate human and mouse libraries, including targeted sublibraries for distinct subpools such as kinases and ribosomal genes (Addgene #51043–51048). Further, improvements to the specificity of sgRNAs have resulted in ‘second generation’ libraries, such as the Brie (Addgene #73632) and Brunello (Addgene #73178) libraries generated by the Doench and Root labs, and the Toronto knockout (TKO) library (Addgene #1000000069) generated by the Moffat lab.