Editing Selfish genetic element (section)

=== CRISPR gene drive and homing endonuclease systems ===
[[CRISPR]] allows the construction of artificial homing endonucleases, where the construct produces guide RNAs that cut the target gene, and homologous flanking sequences then allow insertion of the same construct harboring the Cas9 gene and the guide RNAs. Such gene drives ought to have the ability to rapidly spread in a population (see [[#Gene-drive systems|Gene-drive systems]]), and one practical application of such a system that has been proposed is to apply it to a pest population, greatly reducing its numbers or even driving it extinct.<ref name=":17" /> This has not yet been attempted in the field, but gene drive constructs have been tested in the lab, and the ability to insert into the wild-type homologous allele in heterozygotes for the gene drive has been demonstrated.<ref name=":16" /> Unfortunately, the double-strand break that is introduced by Cas9 can be corrected by [[homology directed repair]], which would make a perfect copy of the drive, or by [[non-homologous end joining]], which would produce "resistant" alleles unable to further propagate themselves. When Cas9 is expressed outside of meiosis, it seems like non-homologous end joining predominates, making this the biggest hurdle to practical application of gene drives.<ref>{{cite journal | vauthors = Champer J, Reeves R, Oh SY, Liu C, Liu J, Clark AG, Messer PW | title = Novel CRISPR/Cas9 gene drive constructs reveal insights into mechanisms of resistance allele formation and drive efficiency in genetically diverse populations | journal = PLOS Genetics | volume = 13 | issue = 7 | pages = e1006796 | date = July 2017 | pmid = 28727785 | pmc = 5518997 | doi = 10.1371/journal.pgen.1006796 | doi-access = free }}</ref>