Many unique features of chronic myelogenous leukemia (CML) make it as a model for studying the development of leukemia in humans. Chronic myeloid leukemia is a disease of the hematopoietic stem cell that progress in a multistep fashion. The biphasic or triphasic clinical course of the disease exemplies the multistep process of tumor progression from the indolent chronic phase to a more aggressive and terminal blast crisis. CML was the first neoplastic disease shown to be associated with consistent karyotypic abnormality now known as the Philadelphia (Ph) chromosome. The result of the Philadelphia chromosome translocation t(9;22)(q34:q11) is the transposition of the c-abl oncogene from chromosome 9 to chromosome 22, where it is fused with part of the her gene. The translocation generates a new hybrid bcr-abl gene which plays a crucial role in the pathogenesis of CML. Presently, CML is perhaps the best understood cancer in humans and the model of oncogenesis mediated by the Ph chromosome translocation is one of the best-characterized example of gene activation in leukemia.