CRISPR, short for clustered regularly interspaced short palindromic repeats, are short DNA repeats found in prokaryotic genomes, characterized by regular spacing between recurring units. These repeats are widely distributed among prokaryotic genomes, suggesting a biological function. The intervening sequences between CRISPR units are believed to originate from foreign genetic elements, such as bacteriophages and conjugative plasmids. These extrachromosomal elements fail to infect the specific spacer-carrier strain, implying a relationship between CRISPR and immunity against targeted DNA. Bacteriophages and conjugative plasmids are involved in prokaryotic population control, evolution, and pathogenicity. The presence of specific spacers could influence these biological traits. CRISPR loci can be visualized as mosaics of a repeated unit, separated by sequences that were present elsewhere in the cell at some point. The study shows that CRISPR spacers derive from preexisting sequences, either chromosomal or within transmissible genetic elements. The highest identities of spacers are with genetic elements, including chromosomes, bacteriophages, and conjugative plasmids, of strains closely related to the one containing the spacer. These targeted viruses are unable to infect the spacer-carrier cell but succeed with closely related strains lacking the specific CRISPR spacer. Plasmids efficiently transferred among various species in the same phylogenetic group cannot be stably maintained in members with a CRISPR spacer matching a sequence in the replicon. The relationship between CRISPR and immunity against targeted foreign DNA is discussed in relation to its functional and evolutionary significance. The study analyzed CRISPR spacers from 67 strains representing 36 genera of prokaryotes. Significant similarities to known sequences were found for 88 spacers from 4 strains of Archaea, 12 strains of Gram-negative bacteria, and 9 strains of Gram-positive bacteria. The spacers matched genes corresponding to bacteriophages, plasmidic DNA, and chromosomal DNA not directly related to foreign genetic elements. The analysis of Sulfolobus CRISPR spacers showed that the spacers showed similarity to known sequences within extrachromosomal genetic elements of Sulfolobus, either SIRV viruses or the conjugative plasmid pNOB8. The study concludes that CRISPR spacers derive from foreign genetic elements and are involved in immunity against targeted DNA.CRISPR, short for clustered regularly interspaced short palindromic repeats, are short DNA repeats found in prokaryotic genomes, characterized by regular spacing between recurring units. These repeats are widely distributed among prokaryotic genomes, suggesting a biological function. The intervening sequences between CRISPR units are believed to originate from foreign genetic elements, such as bacteriophages and conjugative plasmids. These extrachromosomal elements fail to infect the specific spacer-carrier strain, implying a relationship between CRISPR and immunity against targeted DNA. Bacteriophages and conjugative plasmids are involved in prokaryotic population control, evolution, and pathogenicity. The presence of specific spacers could influence these biological traits. CRISPR loci can be visualized as mosaics of a repeated unit, separated by sequences that were present elsewhere in the cell at some point. The study shows that CRISPR spacers derive from preexisting sequences, either chromosomal or within transmissible genetic elements. The highest identities of spacers are with genetic elements, including chromosomes, bacteriophages, and conjugative plasmids, of strains closely related to the one containing the spacer. These targeted viruses are unable to infect the spacer-carrier cell but succeed with closely related strains lacking the specific CRISPR spacer. Plasmids efficiently transferred among various species in the same phylogenetic group cannot be stably maintained in members with a CRISPR spacer matching a sequence in the replicon. The relationship between CRISPR and immunity against targeted foreign DNA is discussed in relation to its functional and evolutionary significance. The study analyzed CRISPR spacers from 67 strains representing 36 genera of prokaryotes. Significant similarities to known sequences were found for 88 spacers from 4 strains of Archaea, 12 strains of Gram-negative bacteria, and 9 strains of Gram-positive bacteria. The spacers matched genes corresponding to bacteriophages, plasmidic DNA, and chromosomal DNA not directly related to foreign genetic elements. The analysis of Sulfolobus CRISPR spacers showed that the spacers showed similarity to known sequences within extrachromosomal genetic elements of Sulfolobus, either SIRV viruses or the conjugative plasmid pNOB8. The study concludes that CRISPR spacers derive from foreign genetic elements and are involved in immunity against targeted DNA.