The Holliday junction. The structure of the Holliday junction is highly variable, being adaptable to its biological function in recombination and to applications in biomolecular engineering. This image shows the how the junction extends from simple schematics to crystal structures as DNA only and in protein complexes. In addition, the junction has been exploited as an element in the design of 2-D and 3-D lattices in crystal engineering and more complex images and shapes through DNA origami. In this issue of Biochemical Society Transactions, P. Shing Ho reviews some interesting recent research on the Holliday junction; for details see pages 1149–1158.