Sera from rats bearing transplantable tumors induced by murine C-type tumor viruses contain an inhibitor of the activity of the viral RNA-dependent DNA polymerase. The inhibitor is shown to be an immunoglobulin (IgG) directed against the enzyme. Antiserum made in rabbits against partially purified murine leukemia virus polymerase also inhibits the polymerases of other mammalian C-type RNA-containing tumor viruses. Thus, the polymerases from different mammalian tumor viruses are antigenically related.
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