Leukaemia virus infection promotes fibroblast transformation by normal BALB/c mouse DNA

All normal cells are thought to carry genetic information for oncogenic transformation, which, on activation to continuous expression, might make the cell cancerous. The presently known transforming retroviruses contain transforming genes which were probably derived by recombination of a slow oncogenic retrovirus with cellular sequences closely related to these genes. It was recently reported that cellular DNA fragments from normal tissue culture cells could transform mouse fibroblasts in vitro with a low efficiency. High efficiency of transformation was observed in secondary transfections only when high molecular weight DNA from transformed recipient cells was used as the transforming agent. We observed that DNA isolated from different BALB/c mouse organs can transform both NIH/3T3 and BALB/3T3 cells, although at a low frequency. In attempts to increase the initial efficiency of transformation, we have found that preinfection of recipient 3T3 cells with murine leukaemia viruses markedly enhances focus formation by normal BALB/c DNA fragments.