Integration of v-rasH does not necessarily transform an immortalized murine cell line

Plasmids containing v-rasH and Ecogpt were constructed, and used to transfect two established cell lines of mouse origin, NIH3T3 cells and m5S cells. After transfection, most NIH3T3 cells, which are resistant to mycophenolic acid, showed phenotypes characteristic of neoplastic transformation, whereas no mycophenolic acid-resistant m5S cells showed these phenotypes; integration of functionally intact v-rasH in immortalized murine cells is not sufficient for neoplastic transformation in m5S cells. The resistance to the transformation was probably due to a lower level of the v-rasH gene transcripts in m5S cells.