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Cultures of mouse Balb 3T3 fibroblasts exposed to a noncytotoxic dose of ethidium bromide for [16][17][18] hr are unable to produce foci after infection with murine sarcoma virus. Such cultures regain susceptibility to infection when incubated for 6-8 hr in drug-free growth medium. Pretreated but not untreated cultures exhibit sensitivity toward brief (6 hr) exposure to cycloheximide, chloramphenicol, and actinomycin D before infection. Pretreatment with cordycepin inhibits the ability of cultures to produce foci after infection. The recovery of ethidium-bromide-treated cultures requires the synthesis of cellular proteins which may have some important role in the establishment of RNA tumor virus infection.

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