The production of exact copies (clones) of genes or DNA sequences using genetic engineering techniques. It is performed to obtain numerous copies of a particular gene so that, e.g., its expression can be investigated or its product isolated in relatively large amounts. Essentially the target gene(s) is inserted into cloning vectors, such as bacterial plasmids or bacteriophages, which transfer the recombinant DNA to suitable host cells, such as the bacterium E. coli or yeast cells. Alternatively, the target DNA can be inserted directly into the host’s DNA using the techniques of genome editing. Inside the host cell the recombinant DNA undergoes replication; thus, a host cell will give rise to a colony of transgenic cells each containing identical cloned DNA fragments. Only a small fraction of host cells produce colonies that successfully incorporate the target gene, and various methods may be used to identify such colonies, e.g. by screening for a reporter gene that confers resistance to a particular antibiotic or expresses a fluorescent protein. The initial target gene may be selected from a genomic library or complementary DNA library, which is itself created by cloning (see dna library). Alternatively, messenger RNA can be isolated from a tissue or a cell culture, complementary DNA copies made using reverse transcriptase, and the target DNA amplified using specific primers with the polymerase chain reaction. Moreover, DNA sequences or even entire genes can be assembled from their component nucleotides using automated synthesizers according to a predetermined base sequence. Gene cloning enables large quantities of a desired protein product to be produced by transgenic host cells (see expression vector): e.g. human insulin is produced by bacteria containing the cloned insulin gene. See also positional cloning.