A collection of cloned DNA fragments representing the entire genetic material of an organism. (i.e. a genomic library) or just the genes transcribed in particular cells or tissues at a given time (i.e. a complementary DNA library, or cDNA library). Genomic libraries facilitate the screening and isolation of any particular gene. They are created by fractionating the entire DNA of an organism into fragments using restriction enzymes and/or physical methods. These fragments are cloned (see gene cloning) and the host cells containing the recombinant fragments are centrifuged and frozen; alternatively, the phage vectors are maintained in culture. Screening for individual genes in the library can be done using specific DNA probes with the Southern blotting technique or, via their protein products, using western blotting. Alternatively, the target DNA can be identified with primer sequences and amplified using PCR. DNA libraries are thus repositories of raw material for use in genetic engineering. A large genome, such as that of humans, is most conveniently cloned using vectors that can accommodate large fragments of DNA, such as yeast artificial chromosomes, maintained in cell culture. Complementary DNA libraries are created essentially by first isolating the messenger RNA molecules expressed in a particular cell or tissue, and then using reverse transcriptase to produce DNA copies. These are joined to plasmid or phage vectors and cloned to create the library. A cDNA library thus represents only part of the genome, namely that which is being expressed at a particular time by the cell type in question.