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Levels of spontaneous and of C-mitomycine and cyclophosphane-induced chromosome aberrations are determined among the persons not subject to professionally industrial hazards. It has been revealed that in all the experimental series the aberration rate in those, who carried extreme polymorphic variants, was much higher compared to those without these variants in their karyotypes. Among the persons with the above extreme versions, the impact of chromosome ruptures is stronger in the vicinity of the structural heterochromatin. Possible causes of the discovered phenomenon are discussed.

A study was made of the action of glucose or sodium succinate on subpopulations of the Ehrlich-I.Ch.Ph. ascite strain characterised by markers "A1" and "A", resp. After i.p. injection of glucose the amount of "A1"-cell reached 50 and almost 100% on the 5th and 7th day of tumor growth. After the transplantation of "A1"-cells into intact animals, a homogenous cytogenetic feature of subpopulation persisted during 2 passages only. Kinetics studies of a subvariance of the Ehrlich-I.Ch.Ph. tumor containing "A1"-cells show that the tumor growth rate and grade of malignancy slightly differ from those seen in the controls.

A study was made of the effect of dibunol and methyl-N-nitrosourea (MNU) on two tumor cell subpopulations of the Ehrlich-I. Ch. Ph. ascites strain, one of which is characterized with A + B + 2C and A + D + 2C--markers and the other one--with A1 + A2 + 2B + D + C markers. Dibunol that belongs to the class of inhibitors of free-radical processes was shown to bring about changes in cell subpopulations, the mode of changes depending on the dose and regime of treatment. The effect of MNU on the population resulted predominantly in the accumulation of cells with various chromosome aberrations. At early stages of tumor progression, aberrations were more pronounced in cells with marker chromosome "A" than in the cells with 44 chromosomes and markers A1 + A2 + 2B + D + C.

After repeated injections to mice of cell-free fluid the inhibition of growth of ascitic cell number was noticed already on the 5th day after transplantation of tumor. It was found that the inhibition of growth was due to the death of cells having 45 chromosomes, and containing A + B + 2C- and A + D + 2C-markers.

G-banded metaphase chromosomes of Chinese hamster V-79 RJK cells resistant to ethidium bromide (2.5 and 10 mcg/ml) have been analysed. The cells of the first selection step (clone IVerb-2, the 9th passage) revealed definite karyotypical instabilities. Amplifications or, in rare cases, deletions were found in chromosomes Z1 and Z6 which appear to have derived from chromosome 1. The amplified region in chromosome Z6 varied considerably in morphology. The chromosome instability, detected in Z1 and Z6, was reproducible in cells throughout the eight independent clones isolated from clone IVebr-2 under non-selective conditions. The data obtained allow to suggest a genetically conditioned mechanism of the above chromosome instability. In the population of resistant cells on the second step of selection (clone I Vebr-5, the 9th passage) the frequency of the cells with amplification increased up to 100%. The length of amplifications increased in the majority of cells. In the cells of the third step of selection (clone IVerb-10, the 12th passage) with near-tetraploid chromosome composition, besides amplifications some specific rearrangements of chromosomes 2 and 7 (markers Z16, Z17) were revealed. The above rearrangements are indicative of the karyotypical destabilization in the drug resistant cells, and may be evaluated as secondary phenomena casually connected with amplifications found at the earlier steps of selection.