A DNA-binding peptide comprising three zinc-fingers has been engineered to bind specifically to a unique nine-base-pair region of a BCR-ABL fusion oncogene in preference to the parent genomic sequences. Binding to the target oncogene in chromosomal DNA is possible in transformed cells in culture, and results in blockage of transcription. Consequently, murine cells rendered independent of growth factors by the action of the oncogene revert to factor dependence upon transient transfection with a vector expressing the peptide.