Four-way DNA (Holliday) junctions are resolved into duplex species by the action of the junction-resolving enzymes, nucleases selective for the structure of helical branchpoints. These have been isolated from bacteria and their phages, archaea, yeasts and mammals, including humans. They are all dimeric proteins that bind with high selectivity to DNA junctions and generate bilateral cleavage within the lifetime of the DNA–protein complex. Recent success in obtaining X-ray crystal structures of resolving enzymes bound to DNA junctions has revealed how the structural selectivity of these enzymes is achieved.

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