There are two distinct types of cloning: molecular and organismal. Molecular cloning is the removal of a stretch of DNA, usually a gene, from an organism, and its insertion into another piece of DNA, such as a plasmid, to form a substance called recombinant DNA. This recombinant DNA may then be expressed in, or simply carried passively by, another organism, such as bacteria. Organismal cloning, the subject of this entry, is the production of genetically identical organisms and, as such, can be used to produce genetically identical copies of livestock or may be used to produce new members of endangered or even extinct species. It may be especially cost-effective to clone animals that produce therapeutic proteins such as blood clotting factors, thus combining both types of cloning. Cloning is controversial, however, because our understanding of the procedures needed to clone mammals may be applied to human...