TRANSGENICS

For many experiments, methods for the longer-term introduction of genetic material into cells are required to maintain stably the resulting gene expression. One approach for this is to select for cells which have integrated the transfected DNA into their chromosomes. This is called stable transfection, but it normally occurs with a low frequency in mammalian cells. It is therefore usually necessary to use a selectable marker that allows the genetic selection of stably transfected cells. A common example of this is the use of the gene for neomycin phosphotransferase. The drug G418 selectively kills cells not expressing this resistance gene. The neomycin phosphotransferase gene (and an associated promoter) may be introduced on a separate plasmid or genetically incorporated into the plasmid whose integration into the genome is required. Often these approaches lead to the integration of several tandem copies of the plasmids at a single chromosomal site. An alternative approach is to use vectors such as bacterial artificial chromosomes (BACs) that permit independent replication (episomal maintenance) of the DNA in the cells (see Topic H3).