Bromodeoxyuridine does not contribute to sister chromatid exchange events in normal or Bloom syndrome cells

Detection of sister chromatid exchanges (SCEs) requires cells to be pulsed with the thymidine analogue bromodeoxyuridine (BrdU) during DNA replication. However, it is widely believed that BrdU induced SCE formation in normal cells. Furthermore, it has been suggested that the high SCE rates characteristic for Bloom syndrome cells are caused solely by BrdU and are not a biological phenomenon. We used Strand-seq to show that BrdU in fact does not induce SCE formation in either normal or Bloom syndrome cells and that SCEs are a biologically relevant phenomenon and not experimentally induced artefacts.