CML) was the first human malignancy to be associated with a single genetic abnormality, characterized by a reciprocal translocation involving chromosomes 9 and 22 (the Philadelphia chromosome). The fusion gene that results (BCR-ABL) produces a constitutively activated tyrosine kinase that exists in different isoforms depending on BCR break-points. Imatinib mesylate is a highly selective inhibitor of this kinase,