Chronic myelogenous leukaemia (CML) results from the neoplastic transformation of a haematopoietic stem cell. The hallmark genetic abnormality of CML is a t(9;22)(q34;q11) translocation, which was first discovered as an abnormal, small chromosome, named the Philadelphia chromosome. This translocation generates the BCR - ABL fusion gene. The initial chronic phase of this biphasic disease is characterized by a massive expansion of the granulocytic cell lineage, even though most, if not all, haematopoietic lineages can be produced from the CML stem cell. The median duration of the chronic phase is 3-4 years. Acquisition of additional genetic and/or epigenetic abnormalities causes the progression of CML from chronic phase to blast phase. This phase is characterized by a block of cell differentiation that results in the presence of 30% or more myeloid or lymphoid blast cells in peripheral blood or bone marrow, or the presence of extramedullary infiltrates of blast cells.