Chromosomal isolabelling caused by three rounds of synthesis in late replicating regions.

Isolabelling only occurs in CHO cells that have been allowed to replicate for more than 2 but less than 3 cell divisions in the presence of BrdU. The isolabelling is confined to late replicating regions of the chromosomes. The staining patterns obtained indicate that BrdU was incorporated three times in these regions and that the isolabelling did not come from the segregation of label in polynemic chromosomes.