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Conformations of four-way junctions. (Left) Association of DNA strands A (blue), B (green), C (red), and D (yellow) to form a junction (Upper) with four duplex arms extended in a square planar geometry (extended-X form, Lower). (Right) These same strands (Upper) associated to form the stacked-X structure of the junction, with pairs of arms coaxially stacked as double helices related by 2-fold symmetry (Lower).
