Abstract
Clones of thioguanine resistant K-BALB mouse [fibroblast] cells were isolated which were inducible for endogenous type C virus synthesis by cycloheximide and dexamethasone, but not 5-iododeoxyuridine. A comparison of the number of foci formed on [normal rat kidney] NRK and [mouse embryo] SC-1 cells suggested that the xenotropic virus was suppressed. The variants were not defective in the incorporation of thymidine or iododeoxyuridine or deficient in thymidine kinase, but were deficient in hypoxanthine-guanine phosphoribosyltransferase and the incorporation of hypoxanthine into nucleic acid. Because these cells are blocked at some point in the expression of endogenous virus, they may prove useful in establishing the steps involved in chemical activation of virus synthesis.

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