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Conformations of a cruciform structure. Conformations of a cruciform can vary from (A) "unfolded" with 4-fold symmetry to (B) bent, and to (C) "stacked" with 4 chains of DNA in close vicinity. D) Topology of a Holliday junction stabilized by a psoralen cross-linking agent (PDBID 467D). Here, the junction takes the form of an anti-parallel stacked x-structure.
