BLM and Bloom syndrome: We found that BLM−/− RPE-1 cells had a > 10-fold increase in SCEs (Supplementary Fig. 3a), similar to Bloom syndrome patient cells4, Blm−/− mouse embryonic fibroblasts47 and BLM−/− chicken DT40 cells48, thus confirming that these RPE-1 clones are true BLM knockout cells.