Genome instability has been associated with progression of transformed cells to high tumorigenicity. Although genome instability may result from a variety of factors, some studies suggest that DNA in the region of a chromosome rearrangement can subsequently have much higher rates of DNA deletions or gene amplification. One approach to studying the factors that produce these high rates of DNA rearrangement is by analysis of unstable integration sites for DNA transfected into mammalian cells. Integrated sequences commonly show a temporary instability, and at rare locations this instability is continuous and can be observed even after multiple subclonings. These continuously unstable locations undergo DNA amplification of both the integrated sequences and the surrounding cell DNA, and it can occur either at the original site or on episomes after looping out from the chromosome. Because the adjacent cell DNA plays a role in this instability, and the region can be shown to be stable before integration, the results indicate that these recombinational hotspots can be formed de novo by the process of integration. Current studies are attempting to determine which sequences are responsible for the high rates of recombination and whether similar types of event are involved in the instability associated with endogenous cellular genes in cancer cells.