Molecular biology. A Swiss army knife of immunity

Selfish genetic elements are more than a daily nuisance in the life of prokaryotes. Whereas viruses can multiply by reprogramming host cells, or integrate in the host genome as “stowaways,” conjugative plasmids (transferrable extrachromosomal DNA) make cells addicted to plasmid-encoded antitoxin factors, thus preventing their disposal. Bacteria and archaea defend themselves against these invasive elements using an adaptive immune system based on clustered regularly interspaced short palindromic repeats (CRISPRs). On page 816 in this issue, Jinek et al. (1) show how the CRISPR effector enzyme Cas9 from bacteria is directed not by one, but two small RNAs to cleave invader DNA.