Murine sarcoma virus transformation of BALB-3T3 cells: lack of dependence on murine leukemia virus.

Murine sarcoma virus induces foci of morphologically altered cells in BALB/3T3 cultures. Focus formation in mouse cells has been thought to require the presence of a helper, murine leukemia virus, which is present in murine sarcoma virus stocks but by itself does not induce any morphological transformation of mouse cells. The present studies show that early after infection, the titration pattern for murine sarcoma virus in BALB/3T3 cells is "two-hit" since only foci produced by virus spread can be detected. Such foci require the presence of both viruses in the initially infected cell. By seven days the titration pattern is "one-hit" under culture conditions which allow the growth and detection of small foci of transformed cells induced by murine sarcoma virus alone. The "two-hit" titration pattern results from the inability to detect these foci. We conclude that murine sarcoma virus is able to transform mouse cells without requiring murine leukemia virus.