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Two specialized forms of recombination have been recognized as playing fundamental roles in restructuring chromosomes, either by reassorting the genetic information or by adding to it or deleting from it. Site-specific recom bination is one restructuring mechanism. In essence this involves a single reciprocal crossover by a specific breakage-reunion reaction between two very short regions of homology. The enzymes involved are a specialized form of sequence specific topoisomerase. Normally no DNA synthesis is required and, hence, the process can be described as a conservative reaction. Site-specific recombination results either in excision or inversion of a DNA segment if the reaction is intramolecular, or in reciprocal translocations or integration of a circular rep Ii con if the reaction is intermolecular. The best known site-specific recombination systems are integration and excision of bacteriophage}..(1), control …