Co-transfection of normal NIH/3T3 DNA and retroval LTR sequences: a novel strategy for the detection of potential c-onc genes.

Morphologically transformed, tumorigenic cell lines were obtained after co-transfecting normal NIH/3T3 DNA and cloned 3'-long terminal repeat sequences of Moloney leukemia virus (Mo-LTR) onto NIH/3T3 recipient cells. In four such cell lines the malignant phenotype was found to be associated with single and specific Mo-LTR integration sites that were retained after serial passages through NIH/3T3 and rat 208F cells, indicating that Mo-LTR sequences are linked to the activated oncogenes. In one of these clones the activated transforming gene was identified as c-raf, the cellular homologue of a recently described retroviral oncogene. This finding not only demonstrates that the mouse c-raf gene can be activated to exhibit an oncogenic potential but also that the approach chosen in this study is suitable for the detection of potential c-onc genes. In contrast to this clone, the activated transforming genes in other cell lines appear to be different from 19 previously isolated v-onc and c-onc genes. These results demonstrate the potential of the established transformation system for the detection and isolation of previously unidentified c-onc genes.