The BCR-ABL tyrosine kinase inhibitor STI571 (Imatinib) has successfully been introduced in the treatment of patients with CML. However, despite encouraging initial data and high expectations long term results are not available yet, and recent data suggest that resistance against STI571 can occur. Therefore, current studies focus on novel potential drug targets in CML cells. Recently, several anti-apoptotic members of the Bcl-2-family including Mcl-1 have been implicated in the regulation of survival of BCR/ABL+ cell lines and therefore proposed as potential targets. We have examined expression of Mcl-1 in primary CML cells and various BCR/ABL-transformed cell lines. Independent of the phase of disease, isolated primary CML cells expressed Mcl-1 mRNA and the Mcl-1 protein in a constitutive manner. The BCR/ABL-inhibitor STI571 decreased the expression of Mcl-1 in these cells. Correspondingly, BCR/ABL enhanced Mcl-1 promoter activity, Mcl-1 mRNA, and Mcl-1 protein in Ba/F3 cells. BCR/ABL-dependent expression of Mcl-1 in Ba/F3 cells was counteracted by the MEK-inhibitor PD98059, but not by the PI3-kinase inhibitor LY294002. Identical results were obtained for constitutive expression of Mcl-1 in primary CML cells and the CML-derived cell lines K562 and KU812. To investigate the role of Mcl-1 as a survival-related target gene in CML cells, Mcl-1 siRNA or an Mcl-1 antisense oligonucleotide (ASO) were applied. Both the ASO and the siRNA-induced downregulation of Mcl-1 were found to be associated with a substantial decrease in viability of K562 cells. Moreover, the Mcl-1 ASO was found to cooperate with STI571 in producing growth-inhibition in both STI571-sensitive and STI571-resistant K562 cells. Together, our data identify Mcl-1 as a survival factor and novel target in CML. Whether this concept is of clinical significance remains to be determined in forthcoming clinical trials.