Gene amplification and drug resistance in mammalian cells.

: Stepwise selection of cultured hamster and mouse cells in progressively increasing concentrations of methotrexate results in highly resistant cells. The resistance results from increased levels of dihydrofolate reductase, the target of methotrexate action in cells. The elevated levels of this enzyme, in turn, results from selective amplification of DNA sequences coding for the enzyme. In some cell lines, the amplified genes are stable when cells are subsequently grown in the absence of methotrexate; these genes are present and localized to a specific region of a chromosome(s). In other cell lines, the amplified genes are unstable and are localized to extrachromosomal elements. These findings are summarized, and mechanisms for the generation of amplified genes are discussed.