The tyrosine kinase inhibitor STI571 is a promising agent for the treatment of advanced Philadelphia chromosome positive (Ph⁺) acute lymphoblastic leukemia (ALL), but resistance develops rapidly in most patients after an initial response. To identify mechanisms of resistance to STI571, 30 complementary DNAs (including 9 matched samples) obtained from the bone marrow of individuals with Ph⁺ ALL were analyzed by direct sequencing of a 714–base pair region of ABL encoding for the adenosine triphosphate (ATP)–binding site and the kinase activation loop. A single point mutation was found at nucleotide 1127 (GI6382056) resulting in Glu255Lys. This mutation occurred in 6 of 9 patients (67%) following their treatment with STI571 but not in the samples from patients before beginning treatment with STI571. Glu255Lys is within the motif important for forming the pocket of the ATP-binding site in ABL and it is highly conserved across species. In conclusion, Ph⁺ ALL samples resistant to STI571 have a unique mutation Glu255Lys of BCR-ABL.