Small interfering RNAs (siRNAs) were designed to target thebcr-abl oncogene, which causes chronic myeloid leukemia (CML) and bcr-abl–positive acute lymphoblastic leukemia (ALL). Chemically synthesized anti–bcr-abl siRNAs were selected using reporter gene constructs and were found to reduce bcr-abl mRNA up to 87% in bcr-abl–positive cell lines and in primary cells from CML patients. This mRNA reduction was specific for bcr-abl because c-abl and c-bcr mRNA levels remained unaffected. Furthermore, protein expression of BCR-ABL and of laminA/C was reduced by specific siRNAs up to 80% in bcr-abl–positive and normal CD34⁺ cells, respectively. Finally, anti–bcr-abl siRNA inhibited BCR-ABL–dependent, but not cytokine-dependent, proliferation in a bcr-abl–positive cell line. These data demonstrate that siRNA can specifically and efficiently interfere with the expression of an oncogenic fusion gene in hematopoietic cells.