Fanconi D2 (FANCD2) is monoubiquitinated on K561 (FANCD2-Ub) in response to DNA double-strand breaks (DSBs) to stimulate repair of these potentially lethal DNA lesions. FANCD2-Ub was upregulated in CD34+ chronic myeloid leukemia (CML) cells and in BCR-ABL1 kinase-positive cell lines in response to elevated levels of reactive oxygen species (ROS) and DNA cross-linking agent mitomycin C. Downregulation of FANCD2 and inhibition of FANCD2-Ub reduced the clonogenic potential of CD34+ CML cells and delayed BCR-ABL1 leukemogenesis in mice. Retarded proliferation of BCR-ABL1 positive FANCD2-/- leukemia cells could be rescued by FANCD2 expression. BCR-ABL1 positive FANCD2-/- cells accumulated more ROS-induced DSBs in comparison with BCR-ABL1 positive FANCD2+/+ cells. Antioxidants diminished the number of DSBs and enhanced proliferation of BCR-ABL1 positive FANCD2-/- cells. Expression of wild-type FANCD2 and FANCD2(S222A) phosphorylation-defective mutant (deficient in stimulation of intra-S phase checkpoint, but proficient in DSB repair), but not FANCD2(K561R) monoubiquitination-defective mutant (proficient in stimulation of intra-S phase checkpoint, but deficient in DSB repair) reduced the number of DSBs and facilitated proliferation of BCR-ABL1 positive FANCD2-/- cells. We hypothesize that FANCD2-Ub has an important role in BCR-ABL1 leukemogenesis because of its ability to facilitate the repair of numerous ROS-induced DSBs.