Infection of mouse and rat cells by the murine sarcoma virus (Moloney isolate) showed two-hit kinetics for focus production in mouse cells but one-hit kinetics in rat cells. Antiserum added to cultures after infection suppressed focus formation in mouse cells but not in rat cells. These studies suggest that, in rat cells infected with murine sarcoma virus, leukemina virus is not needed for focus formation and that these foci result from proliferation of the transformed rat cell; in mouse cells, on the other hand, leukemia virus is needed as "helper," and focus formation requires spread of virus. The term "defectiveness" then, if used, should not be applied to RNA tumor viruses without qualification for the viral function studied and the cell system employed.
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Parkman et al. (1970) studied this question.
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