Spontaneous and methyl methanesulfonate-induced trifluorothymidine-res
istant mutants in mouse lymphoma L5178Y cells were analyzed using fluo
rescence in situ hybridization with mouse probes specific for chromoso
me 11, on which the tk gene is located, and chromosome 3, as the contr
ol. 76.5% (13/17) of small-colony mutants (thought to be the result of
chromosomal mutation) and 28.6% (4/14) of large-colony mutants (thoug
ht to be the result of gene mutation) showed rearranged chromosome 11.
Of the mutants with abnormal chromosome 11 painting pattern, 5 small-
and 2 large-colony mutants carried clonal aberrations, while the rema
ining 8 small- and 2 large-colony mutants showed mosaic aberrations. M
ost abnormalities in the small-colony mutants involved the distal regi
on of one painted chromosome 11, where the tk(+) gene maps. An increas
e, rather than a decrease, in chromosome 11 material was found in a ma
jority of abnormally painted mutants. On the contrary, no rearrangemen
ts involving chromosome 3 were found in any small- and large-colony mu
tants analyzed except one large-colony mutant, which showed chromosome
rearrangements involving both chromosome 11 and 3. The present study
confirms that the majority of small-colony mutants in L5178Y cells hav
e chromosome 11 rearrangements that can be detected by chromosome pain
ting and that the majority of the chromosomal abnormalities in TFT-res
istant mutants involved complex rearrangements.