Genes may be transiently or permanently introduced into cultured eukaryotic transfer cells without the use of a vector in the strict sense. A eukaryotic gene on a bacterial plasmid, for example, may transiently express its product when transfected into a cell line, even if the plasmid does not replicate in that type of cell. Alternatively, DNA introduced by transfection or microinjection may become stably integrated into the cell’s chromosomal DNA. This process normally requires significant sequence similarity between the incoming DNA and the genome in animal cells but, in plant cells, any supercoiled plasmid can randomly integrate into the genome in a process which is not understood in detail. Such stably transfected cells can be selected by the presence of a drug resistance gene in much the same way as bacterial transformants, and can continue to express protein from foreign genes through many cell divisions.
