The article by Mojica et al. investigates the origins of intervening sequences in CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) regions, which are short DNA repeats found in prokaryotic genomes. CRISPR spacers are regularly spaced and widely distributed among various prokaryotic groups, suggesting a biological function. The authors show that these spacers derive from preexisting sequences, including chromosomal DNA and extrachromosomal elements such as bacteriophages and conjugative plasmids. Notably, these extrachromosomal elements fail to infect the specific spacer-carrier strain, indicating a relationship between CRISPR and immunity against targeted DNA. The study suggests that CRISPR may play a role in prokaryotic population control, evolution, and pathogenicity. The findings highlight the importance of CRISPR in the defense mechanisms of prokaryotes against foreign genetic elements.The article by Mojica et al. investigates the origins of intervening sequences in CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) regions, which are short DNA repeats found in prokaryotic genomes. CRISPR spacers are regularly spaced and widely distributed among various prokaryotic groups, suggesting a biological function. The authors show that these spacers derive from preexisting sequences, including chromosomal DNA and extrachromosomal elements such as bacteriophages and conjugative plasmids. Notably, these extrachromosomal elements fail to infect the specific spacer-carrier strain, indicating a relationship between CRISPR and immunity against targeted DNA. The study suggests that CRISPR may play a role in prokaryotic population control, evolution, and pathogenicity. The findings highlight the importance of CRISPR in the defense mechanisms of prokaryotes against foreign genetic elements.