A library is an amplified collection of DNA representing the total DNA of an organism (genomic library) or of DNA complementary to a preparation of mRNA (cDNA library). Either library can be probed to pull out a specific gene of interest. cDNA libraries are used to clone the expressed version of a gene comprised of the exons. Genomic libraries contain both the exons, introns and regulatory portions of the gene. Bacteriophage lambda and cosmid vectors are used in library construction.

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