CRISPR — a widespread system that provides acquired resistance against phages in bacteria and archaea

CRISPR systems, which are composed of direct repeats that are separated by similarly sized non-repetitive spacers, confer resistance to phages in many bacteria and most archaea. This new antiviral tactic is thought to involve an RNA-interference-like mechanism, and is reviewed in this Progress article. Arrays of clustered, regularly interspaced short palindromic repeats (CRISPRs) are widespread in the genomes of many bacteria and almost all archaea. These arrays are composed of direct repeats that are separated by similarly sized non-repetitive spacers. CRISPR arrays, together with a group of associated proteins, confer resistance to phages, possibly by an RNA-interference-like mechanism. This Progress discusses the structure and function of this newly recognized antiviral mechanism.