["The Wilms' tumor gene (WT1) is aberrantly over-expressed in leukemic cells. Therefore, we wanted to study the effect of small interfering (siRNA) targeting WT1 in leukemic cells and normal CD34-positive cells with regard to proliferation, induction of apoptosis, and cell differentiation. Furthermore, we wanted to evaluate whether the additional use of BCR-ABL siRNA could increase the anti-leukemic effects of WT1 siRNA in chronic myeloid leukemia (CML) cells.", 'We measured WT1 expression by reverse transcription polymerase chain reaction (RT-PCR) in various cell lines and in leukemic cells from patients, then transfected the cells with WT1-specific and BCR-ABL-specific siRNA before carrying out microarray analysis. We used the tunnel assay to measure apoptotic cells.', 'We observed a reduction of WT1 gene expression, measured by real-time RT-PCR, in all studied cell lines: K-562, Kasumi-1, MV 4-11 and NB-4, as well as in cells of AML and CML patients. The results also demonstrated that WT1 siRNA significantly induced apoptosis and inhibited proliferation in MV4-11 cells, NB-4 cells, Kasumi-1 cells (p<0.01) and in K-562 cells (p<0.02) versus controls. In normal CD34-positive cells, the proliferation was only slightly inhibited (by about 20%) and no induction of apoptosis was found. Combined transfection with WT1 and BCR-ABL siRNA together in K-562 cells increased the inhibition of the rate of proliferation and the rate of induced apoptosis compared to transfection with BCR-ABL siRNA or WT1 siRNA alone (p<0.01). We found that most genes involved in cell signaling and protein metabolism were regulated by the WT1 gene in K-562 cells in a microarray analysis.'] Do wT1 and BCR-ABL specific small interfering RNA have additive effects in the induction of apoptosis in leukemic cells?