ABSTRACT
Normal Chinese hamster cells contain two copies of the uridine 5'-phosphate (UMP) synthase gene, which maps on hamster chromosome 4p2. By alternating the selective drug pressure, it is possible to go through multiple cycles of amplification and deamplification in a single cell lineage. To determine whether deamplification was a spontaneous event in the amplified cell population, the UMP synthase activity of the cells was checked following growth of the cells in the absence of selective pressure. Initial amplification produced a cell line in which UMP synthase is stably amplified as a homogeneously staining region (HSR) on M3. Amplification and deamplification in this system are accompanied by formation and deletion of expanded chromosomal regions carrying the amplified copies of the UMP synthase genes. Cells selected in the initial amplification step developed an HSR as determined by G-banding.
