SCE induction is proportional to substitution in DNA for thymidine by CldU and BrdU.

The incorporation of 5-bromo- and 5-chlorodeoxyuridine into cellular DNA and the induction of sister-chromatid exchanges (SCE) was examined with Chinese hamster ovary cells in culture. Although CldU caused 3-5 times more SCE than BrdU at equal extracellular concentrations, the 2 analogs were incorporated into DNA in place of thymidine similarly over a concentration range of 0.5-20 microM. The results show that SCE induction is linearly proportional to substitution for thymidine by both BrdU and CldU. Finally, based on linear extrapolation, a spontaneous level of approximately 6 SCEs per cell is estimated.