Abstract
The phenotypic stability of over 2000 large- and small-colony trifluorothymidine-resistant (TFT res) variants of L5178 Y/ tk +/− -3.7.2C cells has been examined. All except 4 of 488 spontaneously arising small-colony variants analyzed (0.8%) retained the TFT res phenotype when rechallenged with TFT after growth for several generations in its absence. All of 558 spontaneous large-colony variants, and 440 small-colony or 487 large-colony variants arising from 13 different mutagens showed similar stability. These results attest to the completeness of TFT selection in the mouse-lymphoma assay when used at 1 μg/ml in Fischer's medium supplemented with heat-inactivated serum and, together with previous cytogenetic and molecular studies, justify considering essentially all such TFT res variants as stable mutants. The implications of these results for those versions of the mouse lymphoma assay that fail to optimize the recovery and scoring of small-colony mutants is discussed.
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