Imatinib is a protein tyrosine kinase inhibitor that potently inhibits the tyrosine kinase bcr-abl in vitro, at the cellular and in vivo levels. This compound selectively inhibits proliferation and induces apoptosis in bcr-abl positive cell lines, as well as in newly infected leukemic cells from patients with Philadelphia chromosome-positive chronic myeloid leukemia (Rh+) and acute lymphoblastic leukemia. In vivo, the compound exhibits antitumor activity as monotherapy in a bcr-abl-positive tumor cell model in animals.