The relationship between the ‘spontaneous’ frequency of sister-chromatid exchanges (SCE) and tumorigenicity was studied in a series of hybrids between a C57BL melanoma cell line and diploid cells, but no correlation was found between the 2 variables. Hybrids in which malignancy was suppressed and malignant segregants derived from them showed virtually identical SCE frequencies. Variation of SCE frequencies was observed, however, between the different hybrid clones, and most hybrids showed consistently less SCE per chromosome than the corresponding parental cell types did under similar growth conditions. The lower SCE frequencies could neither be related to a higher number of chromosomes in the hybrid nor could they be related to the method of hybrid selection. These findings suggest that cell fusion might have induced epigenetic SCE frequency changes possible in the same way as modulation of SCE frequencies is known to occur in the humam leukocyte series.

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