Clustered regularly interspaced short palindrome repeats (CRISPRs) have spacers of extrachromosomal origin.

@article{Bolotin2005ClusteredRI,
  title={Clustered regularly interspaced short palindrome repeats (CRISPRs) have spacers of extrachromosomal origin.},
  author={Alexander Bolotin and Beno{\^i}t Quinquis and Alexei Sorokin and S Dusko Ehrlich},
  journal={Microbiology},
  year={2005},
  volume={151 8},
  pages={2551-2561}
}
Numerous prokaryote genomes contain structures known as clustered regularly interspaced short palindromic repeats (CRISPRs), composed of 25-50 bp repeats separated by unique sequence spacers of similar length. CRISPR structures are found in the vicinity of four genes named cas1 to cas4. In silico analysis revealed another cluster of three genes associated with CRISPR structures in many bacterial species, named here as cas1B, cas5 and cas6, and also revealed a certain number of spacers that have… 

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