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Traditionally clone selection was based on the disruption of a genetic marker (e.g. blue and white screening based on the disruption of the LacZ gene), plasmid retention was based on antibiotic resistance genes. The development of vectors employing selection modules circumvents these issues and permits growth of bacteria harbouring insert-bearing plasmids only. Typically, the vectors used in these systems express a gene product that is lethal to certain bacterial hosts. The lethal gene is inactivated by insertion of a segment of foreign DNA and therefore, toxicity is relieved. The most efficient technical solution remains the killing of bacteria harbouring an insertless vector, or the selection of bacteria harbouring the recombinant vector, the so-called positive selection. The ccdB technology developed by Delphi Genetics has proven to be a successful approach for constructing positive selection vectors