Consistent with the fact that disruption of any of the BLP100 proteins completely abolishes FANCD2-FANCI monoubiquitylation in vivo and in vitro, it is conceivable that the complete absence of monoubiquitylation activity may consistently result in severe FA phenotypes or prenatal loss, as suggested by our Faap100-KO mouse, compared with mouse models with defects of FANCA, -C, -E, -F, or -G (36–38), and by the severe abnormalities of the fetus with the homozygous FAAP100 mutation in the present study. The gene discussed is FANCD2; the disease is Friedreich ataxia.