Imatinib-responsive and -resistant BCR-ABL1 positive cell lines. A) [3H]-thymidine incorporation after 24 h incubation with imatinib. Results of cell line JURL-MK2 are representative for imatinib-sensitive, results of cell line SUP-B15 are representative for imatinib-resistant cell lines. B) Time-course data confirm resistance of cell line SUP-B15 to imatinib (100 nM) for a period of four days. C) Apoptosis assessed by annexin-V/PI staining. Imatinib (1 μM, 24 h) induced apoptosis in sensitive JURL-MK2 cells, but not in the imatinib-resistant cell line SUP-B15. Control: cells cultivated for 24 h in medium without TKI.