Since it was first recognized, chronic myeloid leukemia (CML) has always represented a unique model to understand the molecular mechanisms underlying the onset and progression of a leukemic process. CML was the first recognized form of cancer to have a strong association with a recurrent chromosomal abnormality, the t(9;22) translocation, which generates the so-called Philadelphia (Ph)-chromosome. Twenty years later, this abnormality was shown to cover a specific molecular defect, a hybrid BCR-ABL gene, strongly implicated in the pathogenesis of the disease through the production of a protein with a constitutive tyrosine-kinase activity. Although we still lack a complete definition of all the transformation pathways activated by Bcr-Abl, the recent introduction into clinical practice of tyrosine kinase inhibitor represents a major breakthrough to the management of CML and, furthermore, promises to usher in molecularly targeted therapy for other types of leukemia, lymphoma and cancer.