DNA cloning or molecular cloning is the process of making multiple identical copies of a particular piece of DNA. Molecular cloning generally uses DNA sequences from two different organisms: the species that is the source of the DNA to be cloned, and the species that will serve as the living host for the replication of the recombinant DNA. Molecular cloning methods are at the heart of many contemporary fields of modern biology and medicine. In a typical DNA cloning procedure, the gene or other DNA fragment of interest is first inserted into a circular piece of DNA called a plasmid. The insertion is performed using enzymes that "cut and paste" the DNA, and it produces a recombinant DNA molecule, or DNA assembled from fragments from multiple sources.
Virtually any DNA sequence can be cloned and amplified, but certain factors can limit the success of the process. Examples of DNA sequences that are difficult to clone are reverse repeats, origins of replication, centromeres and telomeres. Another characteristic that limits the chances of success is the large size of the DNA sequence. Insertions larger than 10 kbp have very limited success, but bacteriophages such as the bacteriophage λ can be modified to successfully insert a sequence up to 40 kbp.