A PRINCIPAL goal of biotechnology is the assembly of novel biomaterials for analytical, industrial and therapeutic purposes. The advent of stable immobile nucleic acid branched junctions1–4 makes DNA a good candidate for building frameworks to which proteins or other functional molecules can be attached and thereby juxtaposed5–7. The addition of single-stranded 'sticky' ends8 to branched DNA molecules converts them into macromolecular valence clusters that can be ligated together1. The edges of these frameworks are double-helical DNA, and the vertices correspond to the branch points of junctions. Here, we report the construction from DNA of a covalently closed cube-like molecular complex containing twelve equal-length double-helical edges arranged about eight vertices. Each of the six 'faces' of the object is a single-stranded cyclic molecule, doubly catenated to four neighbouring strands, and each vertex is connected by an edge to three others. Each edge contains a unique restriction site for analytical purposes. This is the first construction of a closed polyhedral object from DNA.