Three independently-derived, antifolate-resistant Chinese hamster lung cell lines that exhibit low level increases in dihydrofolate reductase (DHFR) activity, i.e., three- to fivefold vs. controls, have been compared with drug-sensitive cells to determine relative DHFR gene content. With a solution hybridization technique that makes use of genomic DNA and a cloned double-stranded Chinese hamster DHFR cDNA probe, it has been found that the enzyme activity increases are associated with an approximately proportionate amplification of DHFR genes. Trypsin-Giemsa staining techniques and hybridizations in situ further show that the amplified DHFR genes are located within abnormally banding regions along chromosome 2q and also suggest that, in each subline, only one chromosome 2 homolog is initially involved in the amplification process.

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