CRISPR-Cas: Adapting to change

Variation in prokaryote adaptive immunity To repel infection by phage and mobile genetic elements, prokaryotes have a form of adaptive immune response and memory invested in clustered regularly interspaced short palindromic repeats and associated proteins (CRISPR-Cas). This molecular machinery can recognize and remember foreign nucleic acids by capturing and retaining small nucleotide sequences. On subsequent encounters, the cognate CRISPR-Cas marshals enzymatic defenses to destroy infecting elements that contain the same sequences. Jackson et al. review the molecular mechanisms by which diverse CRISPR-Cas systems adapt and anticipate novel threats and evasive countermeasures from mobile genetic elements. Science , this issue p. eaal5056