Genetic stability of the sarcoma viruses in murine and avian sarcoma virus‐transformed nonproducer cells

Clonal lines of murine and avion sarcoma virus‐transformed mammalian cells, in which the viruses do not productively replicate, were studied for the occurrence of spontaneous or chemical‐induced morphologic reversion. From over 10,000 single‐cellderived clones of MSV‐transformed nonproducer cells examined, only one morphologic revertant was detected. This cell line contained a normal murine sarcoma viral genome, suggesting that it arose as a result of mutation of a cellular gene involved in expression of the transformed state. The frequency of morphologic reversion of avian sarcoma virus‐transformed mammalian cell lines was also investigated. The studies indicate that both murine and avian sarcoma viruses present in transformed nonproducer cells can exist in a genetically stable form.