This study demonstrates in both stable and inducible BCR-ABL–expressing hematopoietic cells a down-regulation of the major mammalian DNA repair protein DNA-PKcs by BCR-ABL. Similar results were found in BCR-ABL CD34⁺ cells from patients with chronic myelogenous leukemia (CML). DNA-PKcs down-regulation is a proteasome-dependent degradation that requires tyrosine kinase activity and is associated with a marked DNA repair deficiency along with increased sensitivity to ionizing radiation. The conjunction of a major DNA repair deficiency and a resistance to apoptosis, both induced by BCR-ABL, provides a new mechanism to explain how secondary genetic alterations can accumulate in CML, eventually leading to blast crisis. The down-regulation of DNA-PKcs was reversible in CD34⁺ CML cells suggesting that this approach might offer a novel and powerful therapeutic strategy in this disease, especially to delay the blast crisis.