The conceptual foundation for DNA nanotechnology was first laid out by Nadrian Seeman in the early 1980s. A number of natural branched DNA structures were known at the time, including the DNA replication fork and the mobile Holliday junction, but Seeman's insight was that immobile nucleic acid junctions could be created by properly designing the strand sequences to remove symmetry in the assembled molecule, and that these immobile junctions could in principle be combined into rigid crystalline lattices. The first theoretical paper proposing this scheme was published in 1982, and the first experimental demonstration of an immobile DNA junction was published the following year. Seeman developed the more rigid double-crossover (DX) motif, suitable for forming two-dimensional lattices, demonstrated in 1998 by him and Erik Winfree. In 2006, Paul Rothemund first demonstrated the DNA origami technique for easily and robustly creating folded DNA structures of arbitrary shape. This method allowed the creation of much larger structures than were previously possible, and which are less technically demanding to design and synthesize. The synthesis of a three-dimensional lattice was finally published by Seeman in 2009, nearly thirty years after he had set out to achieve it.