Large origami from a single strand Nanostructures created by origami-like folding of nucleic acids are usually formed by base-pairing interactions between multiple strands. Han et al. show that large origami (up to 10,000 nucleotides for DNA and 6000 nucleotides for RNA) can be created in simple shapes, such as a rhombus or a heart. A single strand can be folded smoothly into structurally complex but knot-free structures by using partially complemented double-stranded DNA and the cohesion of parallel crossovers. The use of single strands also enables in vitro synthesis of these structures. Science , this issue p. eaao2648