A cDNA library consists of a collection of DNA that is complementary to an organism’s mRNA. The cDNA (complementary DNA) is synthesized by reverse transcription of mRNA. The major advantage of a cDNA library is that it contains only fragments from actively transcribing genes. Another advantage is that the cloned sequences are not disrupted by introns, which would hamper the ability to produce eukaryotic protein in bacteria as most bacteria lack the mechanism necessary for eliminating the introns.