A cDNA library is a combination of cloned cDNA (complementary DNA) fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of the organisms. cDNA is produced from fully transcribed mRNA found in the nucleus and therefore contains only the expressed genes of an organism. Similarly, tissue specific cDNA libraries can be produced. In eukaryotic cells, the mature mRNA is already spliced, hence the cDNA produced lacks introns and can be readily in a bacterial cell. While information in cDNA libraries is a powerful and useful toll since gene products are easily identified, the libraries lack information about enhancers, introns, and other regulatory elements found in a genomic DNA library.