Although previous studies attributed a pro-survival role to USP9X in human cancer, how USP9X affects B-cell acute lymphoblastic leukemia (B-ALL) remains unclear. Here, we found that USP9X is overexpressed in B-ALL cell lines and human patients. We investigated the role of USP9X in B-ALL and found that USP9X knockdown significantly reduced leukemic cell growth and increased spontaneous apoptosis, thereby improving survival in immunodeficient mice. These effects are partially mediated by the intrinsic apoptotic pathway, as we found that USP9X-knockdown leukemic cells displayed MCL1 down-regulation, with decreased BCL-2/BCL-XL levels and increased BAX levels. In addition, we demonstrated that USP9X inhibition negatively regulates mTORC1 activity toward its substrate S6K1. Clinically, USP9X inhibition sensitized glucocorticoid-resistant ALL cells to prednisolone; this observation reveals a potential avenue for improving the treatment of drug-resistant relapses. Collectively, our findings suggest that the combination of USP9X targeting and glucocorticoids treatment has attractive utility in B-ALL. This approach represents a potential strategy for promising combination therapies for lymphoid malignancies. (C) 2015 Elsevier Inc. All rights reserved.