Genetic experiments using a dominant-negative form of human telomerase (DN-hTERT) demonstrated that telomerase inhibition can result in telomeric shortening followed by proliferation arrest and cell death by apoptosis. Neoplastic cells from telomerase RNA null (mTERC-/-) mice showed enhanced chemosensitivity to doxorubicin or related double-strand DNA break (DSB)-inducing agents. Telomerase dysfunction, rather than telomerase inhibition, is proposed to be the principal determinant governing chemosensitivity specifically to DSB-inducing agents. We observed that imatinib and vincristine (VCR), in addition to DSB-inducing agents, also enhanced chemosensitivity in telomestatin-treated K562 cells. This observation suggests that combined use of telomerase inhibitors and imatinib or other chemotherapeutic agents may be a very useful approach to treatment of BCR-ABL-positive leukemia.