The locations of observed breaks in three pairs of painted chromosomes from radiation-exposed and unexposed human peripheral blood lymphocytes are described. No difference in the observed breakpoint locations was seen from people exposed at Chernobyl, from healthy controls or from blood exposed to 2 Gy137 Cs in vitro. However, the distribution of observed breaks within the painted chromosomes was not random. Fewer breaks and rearrangements were observed near the ends of the chromosome arms. Three explanations for these findings were considered: cell selection, non-random efficiency of detection and non-random breakage or repair. Cell selection does not appear to be plausible because the distribution of observed breaks induced in vitro is not different from those induced in vivo. Non-random efficiency of detection is not supported by the data. Non-random breakage or repair appears to be the most likely explanation, although the mechanism(s) by which this occurs is unknown.

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