Library of genetic material, similarly to common libraries, store information. DNA library of a certain organism is made by cleaving the whole genome with restriction enzymes. Then fragments are spliced with vectors cleaved by the same enzyme and the latter are introduced to the host cells. For bacteria by using certain plasmids and antibiotics it is possible to identify cells that taken the plasmids. Cells of the library can be stored and when needed desired gene can be separated by using probes.

Information about RNA can also be kept in a library. Although RNA is rather unstable and inconvenient for investigation, it is possible to store it’s copy – cDNA. Complementary DNA (cDNA) is synthesized by reverse transcriptase (it can be found in certain viruses that keep their genetic information in RNA and make a DNA copy before introducing it to the host DNA). cDNA libraries are especially useful when information about genes that undergo expression is needed. Copies of mRNA (which reflects active genes) present in a given cell can be made.