How does stable transfection of a shRNA-encoding plasmid work?
FAQ ID -2779

Most mammalian expression plasmids do not replicate in mammalian cells. They are eventually lost due to dilution caused by cell growth and division. The use of a selectable marker (such as antibiotic resistance), on the same or another plasmid (at one-tenth the amount), enables the investigator to select for the very rare incorporation of the plasmid DNA into the cell line genome. Once integrated, the plasmid sequence is replicated with the rest of the genome and is stably transmitted to both daughter cells, resulting in the long term expression of the shRNA of interest.