We found that while FANCD2-deficient cells were hypersensitive to MMC, they exhibited no detectable hypersensitivity to topotecan, arguing that the mechanisms of FA cell killing by these two drugs are inherently different (Supplementary Figure 9e, f; data are for FANCD2−/− cells complemented with an empty plasmid vector or vector containing WT FANCD2). The gene discussed is FANCD2; the disease is Friedreich ataxia.