The idea that DNA base pairing could direct the crystallization of useful materials is a tempting one for nanotechnologists. Now - over ten years after it was first shown that DNA attached to nanoparticles can influence their assembly - two groups have put this concept into practice. Park et al. demonstrate that the DNA molecules attached to gold nanoparticles, and DNA molecules used to link them, can be selected to ensure that the nanoparticles self-assemble into either face-centred cubic or body-centred cubic crystals. The cover graphic, by Cole Krumbholz, is a close-up of the latter. Nykypanchuk et al. identify the requirements for DNA design and the crystallization conditions that allow the reversible formation of body-centred cubic crystals, with nanoparticles occupying just a few percent of a lattice volume. As discussed in News & Views, these developments might make it possible to create ordered and tunable 3D nanoscale architectures relevant for photonic and magnetic applications, biomedical sensing, and information or energy storage.