CRISPR/Cas9-based DNA targeting has quickly become a leading tool in the fields of synthetic biology and genome engineering. It exploits the ability of a bacterial endonuclease, Cas9, guided by an RNA molecule, to target virtually any matching DNA sequence of interest for binding and/or cleavage. A study published in this issue of Nature reports the use of single-molecule and bulk biochemical experiments to reveal the mechanism by which RNA-guided Cas9 locates unique 20-base-pair sequences within DNA genomes, which can be billions of base pairs long. The results highlight the role of a trinucleotide protospacer adjacent motif (PAM; yellow in the cover image) in recruiting Cas9RNA complexes to potential DNA target sites, and in catalytically activating the nuclease (outlined in brown). Target DNA sequences are recognized via a zip-up� mechanism, where the sequential formation of RNADNA base pairs (red) offsets the energetic cost of unwinding the DNA double helix (purple and blue). In addition to its relevance for gene manipulation, this work reveals how DNA is interrogated by Cas9RNA in its role as an effector of adaptive immunity in bacteria. Cover: K. C. Roeyer.