The tumorigenic potential of pre-B-cells at different stages of Abelson murine leukemia virus-induced transformation was determined. Cell lines with low growth potential in liquid culture were found (a) to have a dose-dependent growth requirement for conditioned medium obtained from bone marrow cultures, (b) to have low colony-forming ability in semisolid medium in the absence of conditioned medium, and (c) to be nontumorigenic when inoculated into syngeneic mice. Culture of the factor-dependent cells in vitro leads to the emergence of factor-independent variants, which eventually dominate the population by overgrowth. Cell lines that acquired a factor-independent phenotype were able to form colonies in semisolid medium and form tumors when inoculated into syngeneic mice. These results suggest that Abelson murine leukemia virus is sufficient to initiate transformation in the infected cell but that an additional genetic alteration is needed to confer tumorigenicity.

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This study was supported by Public Health Service Grants AI-11851 and CA-234043 from the NIH. G. J. P. was supported by Cancer Immunology Training Program Grant 5-T32-09082 from the National Institutes of Health.

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