Target site selection in CAST systems Exciting genomic engineering possibilities exist for natural integration systems called transposons, which have co-opted CRISPR/Cas systems. An unexplained feature of these systems involves how they direct insertions in a single orientation at a precise distance from the programmed target sequence. Park et al . show that orientation information is communicated to the transposase, TnsB, using the unidirectional growth of a helical filament made up of an AAA+ protein, TnsC. ATP hydrolysis trims the filament to a minimal unit that is marked by TniQ and defined by the Cas12k protein to provide spacing information. This finding may help future engineering of these systems for therapeutic applications. —DJ