Evidence that sister chromatid exchanges and chromatid breaks are two independent events.

The relative frequencies of sister chromatid exchanges (SCE) and chromatid breaks in BrdU (5-bromodeoxyuridine) - sensitive site (lq22 leads to lq23) in Chinese hamster cells after BrdU incorporation were studied. The results show that chromatid breaks do not follow the "exchange hypothesis" and provide evidence that chromatid breaks and SCEs are two independent events despite some common features.