Revertants of Mouse Cells Transformed by Murine Sarcoma Virus: flat Variants without a Rescuable Sarcoma Virus from a Clone of BALB/3T3 Transformed by Kirsten MSV

* Viral Leukaemia and Lymphoma Branch, National Cancer Institute Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20014, U.S.A. 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. Received 20 May 1974; accepted 3 July 1974.