Induction of apoptosis requires inhibition of the Bcr-Abl kinase to a certain threshold in Baf3/BCR-ABL-r1 cells. Whole cell lysates of Baf3/BCR-ABL-r1 cells were analyzed after 48 hours of exposure to gradually increasing doses of imatinib (x-axis). Western blots using antiphosphotyrosine (α-pY) and antiabl (α-abl) antibodies were performed (insert in left panel). Bcr-Abl kinase activity was derived after densitometry (OD, y-axis) and correction for protein expression (pBcr-Abl/Bcr-Abl ratio). Fractions of the same cultures were analyzed for caspase-3 activation (right panel). Data points represent means of 4 independent experiments ± SD. Statistical analysis using a 1-sided t test revealed 2 μM imatinib as the first dose level with increased apoptosis above baseline (*P < .05).