leukaemialymphomaresearch.org.uk A DNA library is a collection of cloned DNA fragments stored in host cells (usually bacteria), which constitute an organism’s entire genome. There are also cDNA (complementary DNA) libraries that are made through the reverse transcription of messenger RNA isolated from an organism. Such cDNA libraries give insight of which genes are actively transcribed in an organism; one drawback is that in cDNA libraries not all genes are represented. DNA libraries are created by cutting the DN A using mechanical methods or with a restriction endonuclease, and then cloning the fragments into vectors to produce recombinant DNA. Vectors are ‘delivery vehicles’ that can carry inserted DNA into host cells; there are several types, which differ in their insert capacity (the amount of DNA that can be inserted). Some of these vectors are: plasmids, bacterial artificial chromosome (BAC), cosmids, yeast artificial chromosome (YAC), and bacteriophage λ [1]. Plasmids