It works for cloning too
One of the major drawbacks in DNA cloning is the scarcity of the insertion event of the DNA insert into the plasmid. Typically, less than 10% of the vectors circularize with an insert. From the early development of molecular cloning, identifying vectors with an insert has always been a frustrating and time-consuming step for the investigator.
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. The flexibility of the ccdB selection was proven by the system of vectors for different cloning applications based on Delphi Genetics' ccdB technology sold by the licensee of this technology, Invitrogen Inc. Most of the early commercial vector versions were focused on the cloning of PCR generated inserts because the cloning of such inserts leads to an appreciable drop in cloning efficiency (and thus increases the background). This is especially true when a thermostable polymerase presenting a proofreading activity that generates blunt-ended fragments is used.