Clonal subline of Kirsten murine sarcoma virus (Ki-MSV)-transformed non-producer BALB/3T3 (Ki-BALB) cells spontaneously produced flat variants with some properties of non-transformed cells at frequencies of 0.01 to 0.001. Such variants were epithelioid, contact-inhibited, grew to low saturation density and exhibited variable cloning efficiences in soft agar. They demonstrated no murine leukaemia group-specific antigen(s), no reverse transcriptase activity, and no infectious virus, but were agglutinable by concanavalin A. Murine leukaemia virus (MuLV) was induced after 5-iododeoxyuridine (IUDR) treatment from both the flat variants and from the parental Ki-BALB cells. However, Ki-MSV could not be rescued from these flat variants by superinfection with MuLV, nor induced by treatment with IUDR. The state of reversion was stable, revealing no back-transformation during 30 to 40 subcultures. The tumourigenicity of the flat variants in BALB/c mice was markedly reduced (actually undetectable) compared to that of the Ki-BALB cells. All flat variants were susceptible to MSV and MuLV infection. The loss of the transformed phenotype in spontaneous flat variants was associated with a decrease in chromosome number to a level similar to BALB/3T3 cells. These observations suggest that these revertants may have lost some or all of the Ki-MSV genome.


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