DNA, present in almost every cell, is increasingly being used as a building material to construct tiny, but sophisticated structures such as autonomous ‘DNA walkers’ that can move along a microparticle surface, fluorescent labels for diagnostic applications, ‘DNA boxes’ that serve as smart drug-delivery vehicles programmed to open up at disease sites to release their therapeutic content, or programmable factories for nanoparticles of defined sizes and shapes for new optical and electronic applications.