In the early 1970s, Stanley Cohen et al. reported the production of a new, autonomously replicating DNA molecule that had been constructed in vitro.1 It was produced by digesting separate tetracyclineresistant (pSC101) and kanamycin-resistant (pSC102) DNA fragments with the restriction endonuclease EcoRI and then covalently linking the two fragments with DNA ligase. Transformation of competent Escherichia coli with the resulting ligation mixture yielded clones with resistance to both tetracycline and kanamycin localized on a single contiguous DNA molecule (pSC105). is was the rst published report of recombinant DNA technology. Since that time, recombinant DNA molecules have been used to produce a variety of both small molecules and proteins in host organisms ranging from prokaryotic microbes to animal cells. Such productivity is based on the promiscuous nature of molecular biologybased on a lack of delity to the genome-and the demonstration by Cohen and colleagues that these recombined genes could come from a variety of sources and be readily transcribed in a bacterial host.1-3 us, the biotechnology revolution that began with these rst experiments was predicated on the ability to introduce and maintain heterologous DNA in a host organism. e carriers of foreign DNA, or vectors, may be thought of as “articial chromosomes,” enabling the in vivo expression of genes that are maintained separately from the genome.