ΔFANCB DT40 cells have a defective FA pathway because they do not exhibit FANCD2 monoubiquitination after MMC or HU treatment (Figure 1D, lanes 5 and 6), which is in agreement with previous work in FA patient cells that lack functional FANCB (Meetei et al., 2004). Here, FANCD2 is linked to Friedreich ataxia.