crispr (clustered regularly interspaced short palindromic repeats) is an acronym for dna loci that contain multiple, short, direct repetitions of base sequences. each repetition contains a series of base pairs followed by the same or a similar series in reverse and then by 30 or so base pairs known as "spacer dna". crisprs are found in the genomes of approximately 40% of sequenced bacteria and 90% of sequenced archaea. crisprs are often associated with cas genes which code for proteins that perform various functions related to crisprs. the crispr-cas system functions as a prokaryotic immune system, in that it confers resistance to exogenous genetic elements such as plasmids and phages and provides a form of acquired immunity. crispr spacers recognize and silence exogenous genetic elements in a manner analogous to rnai in eukaryotic organisms.