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Methods for gene transfer vary widely in their relative efficiencies. At an appropriate multiplicity of infection, virtually all the cells in a population can be infected with a recombinant retrovirus. All other means of stable transformation are less efficient. To facilitate the isolation of cells stably transformed with the DNA of interest, a gene encoding a dominant selectable marker is usually included in the transfection protocol. Such a gene encoding a dominant selectable marker can be provided either in cis or in trans to the other transferred gene. Both approaches work well. When the selectable marker is provided in cis, the rate of phenotypic co-transformation—the rate at which both the selectable marker and non-selectable gene are expressed in the same transfected cell—is virtually 100 percent. When the selectable marker and non-selectable gene are provided in trans, the phenotypic co-transformation rate is approximately 80 percent and if three different DNAs are transfected, including one selectable marker, the phenotypic co-transformation rate for all three is approximately 60 percent (Kriegler et al., 1983). |
