Cytotoxicity and DNA double strand breaks (DSBs) were studied in HeLa cells treated With Sulforaphane (SFN), a well-known chemo-preventive agent. Cell survival was impaired by SFN in a concentration and treatment time-dependent manner. Both constant field gel electrophoresis (CFGE) and gamma-H2AX assay unambiguously indicated formation of DSBs by SFN, reflecting the cell Survival data. These DSBs were predominantly processed by homologous recombination repair (HRR), judging from the SFN concentration-dependent manner of Rad51 foci formation. On the other hand, the phosphorylation of DNA-PKcs, a key non-homologous end joining (NHEJ) protein, was not observed by SFN treatment, suggesting that NHEJ may not be involved in DSBs induced by this chemical. G2/M arrest by SFN, a typical response for cells exposed to ionizing radiation was also observed. Our new data indicate the clear induction of DSBs by SFN and a useful anti-tumour aspect of SFN through the induction of DNA DSBs. (C) 2008 Elsevier Inc. All rights reserved.