Amplified dihydrofolate reductase genes are localized to a homogeneously staining region of a single chromosome in a methotrexate-resistant Chinese hamster ovary cell line.

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Abstract
Methotrexate-resistant Chinese hamster ovary cells selected for high resistance by progressive increments of methotrexate in the culture medium have levels of dihydrofolate reductase(tetrahydrofolate dehydrogenase, 7,8-dihydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) 200 times that of sensitive cells and a corresponding increase in the number of copies of the dihydrofolate reductase gene. The resistant cells contain an expanded region on the 2nd chromosome (homogeneously staining region) that is not present in sesitive cells. In situ hybridization of DNA complementary to dihydrofolate reductase mRNA shows that the dihydrofolate reductase genes are specifically localized to the homogeneously staining region of this chromosome in the resistant cells.