Nbs1, a member of the DNA repair/checkpoint activation complex Mre11/Rad50/Nbs1, is phosphorylated by ATM in response to the presence of DNA double strand breaks (DSBs) resulting in the activation of the S phase checkpoint. Here we show that the BCR/ABL tyrosine kinase, as well as growth factors (IL-3, GM-CSF), stimulate the expression of Mre11 and Nbs1, but not Rad50. This effect is dependent on the kinase activity of BCR/ABL protecting Mre11 and Nbs1 from caspase-dependent, but not proteasome-dependent degradation. Cells expressing the BCR/ABL kinase are resistant to chemotherapeutic agents, including mitomycin C (MMC). Western analysis showed enhanced phosphorylation of Nbs1 on Serine 343 in MMC-treated BCR/ABL leukemia cells (CML patient cells and leukemic cell lines) in comparison to normal cells in the presence of growth factors. Immunofluoresence studies demonstrated an increase of γ-H2AX nuclear foci (DSBs indicator) and pNbs1 nuclear foci in BCR/ABL cells after MMC treatment in comparison to parental counterparts. In addition, BCR/ABL-positive cells displayed a higher percentage of colocalization of γ-H2AX and p-Nbs1 implicating the presence of p-Nbs1 at the DSBs. Clonogenic assays performed after down regulation of Nbs1 in BCR/ABL positive cells using siRNA showed increased sensitivity to MMC. Specifically, the expression of the Nbs1-S343A phosphorylation-less mutant also decreased resistance in BCR/ABL cells to MMC. The radioresistant DNA synthesis (RDS) assay showed that MMC-treated CML patient cells, BCR/ABL-transformed cell lines and normal counterparts displayed an inhibition of DNA synthesis associated with transient accumulation of the cells in S phase. Expression of Nbs1-S343A mutant caused a significant decrease in the accumulation of BCR/ABL leukemia cells in S phase after MMC treatment, whereas cells transfected with both the empty construct and wild-type Nbs1 displayed S phase accumulation. Surprisingly, Nbs1-S343A mutant did not affect the ability of normal cells to accumulate in S phase in response to MMC. Altogether, we hypothesize that enhanced phosphorylation of Nbs1 on S343 leads to increased resistance to genotoxic agents in BCR/ABL leukemia cells by prolonging the S phase checkpoint and allowing longer time for the repair of excessive DNA damage.