of the DNA to permit selection for its presence either in eukaryotic cell lines or in the bacteria in which the DNA was mass-produced.

Retrovirus Vectors

Retroviruses are infectious elements that replicate by a unique process involving copying of the viral RNA genome into DNA (a process called reverse transcription) followed by its specific and stable introduction into host cell DNA (integration). The integrated DNA then can be expressed using the normal transcriptional machinery of the cell. Retroviruses commonly are used to introduce genes of interest into cells in culture or into somatic tissue in experimental animals (Miller, 1997). They also have been used for germline modification of fish (Amsterdam et al., 1997), mollusks (Lu et al., 1996) chickens (Thoraval et al., 1995), mice (Soriano et al., 1986), and, more recently, cattle (Chan et al., 1998). To make a retrovirus vector, a DNA construct containing the gene of interest is flanked by sequences necessary for replication as a virus. These sequences include transcriptional promoters in the long terminal repeats (LTR’s), which flank the integrated DNA, or provirus. Signals necessary for packaging of the transcript in virions (virus particles), for reverse transcription, and for integration of the resulting DNA also must be included. Introduction of such a DNA construct into cells that express viral proteins, but that are incapable of making infectious virus (i.e., helper, or packaging, cells), leads to the creation of infectious virions containing an RNA copy of the gene of interest. After infection of cells with such virions, the RNA is copied into DNA and integrated at random sites in the cell genome. Again, selectable markers often are included in the construct to select cells containing the desired virus construct.

Transposons

Transposons are DNA elements that (in the presence of the appropriate gene products, or transposases) can transfer their information from one site to another in the same cell. A variety of transposons have been found in insects (Handler, 2001) and fish (Ivics et al., 1997), and some are routinely used as vectors for the generation of transgenic insects (Braig and Yan, 2002). No active transposons of these types have been observed in mammals, although the human genome contains thousands of copies of a DNA sequence related to the mariner transposon of Drosophila (Lander et al., 2001), suggesting that there might have been active elements in our recent evolutionary history. Nevertheless, several recent reports suggest that naturally-occurring transposons found in insects, or even bacteria, might provide a useful and



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