CML) is characterised by the occurrence of the Philadelphia (Ph) chromosome (9/22 translocation) and the formation of a fusion protein--the BCR-ABL transcript with constitutive activation of the BCR-ABL tyrosine kinase and consequent changes in the intracellular signal transduction, which is responsible for the deregulated myeloid cell proliferation. STI571 (signal transduction inhibition number 571) is a potent and selective inhibitor of the BCR-ABL tyrosine kinase