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.