A comparison is made between the incidences of sister chromatid exchanges (SCE) per chromosome and group of chromosomes and breakage, visible at metaphase like open gaps, breaks, and breaks involved in chromatid interchange formation (CI) in Bloom's syndrome. It can be shown that the two levels of breakage SCE and CI are not correlated as to the locations. The discussion deals with possible interpretations of preferential breakage and reunion at certain homologous chromosomes and the difficulties today to understand SCEs.

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