The phenomenon of sister chromatid exchange (SCE) is an interesting genetic event in metaphase chromosomes, even though its exact mechanism remains unknown. The fact that SCE can take place, whether 'spontaneously' or induced by various agents, is in itself important, for such an event, involving damage and possible repair of bilateral loci in chromosomes, presents opportunities for modification of the cell characters (e.g. oncogene activation). BS B- lymphoblastoid cell lines with high SCE levels were highly susceptible to malignant transformation by chemical carcinogenesis, in which cancer antigens are expressed on the cell surface with the disappearance of immunoglobulin (Ig), when the number of SCE exceeded 140 SCE per cell (over the baseline of 70 SCE per cell). The existence of an increased level of SCE which might cause unequal recombination between either sister chromatids or chromatids of homologous chromosomes may reflect phenomena secondary to malignant transformation or may have played a key role in initiation and maintenance of expression of the oncogenes and the antigens associated with cancers.

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