Abstract
Although chronic myeloid leukemia (CML) is now defined on the basis of the presence of the BCR-ABL1 fusion gene, which may or may not be the initial genetic event that triggers the inappropriate expansion of the myeloid cell mass, CML, similar to other leukemias, is in fact clinically heterogeneous. The biological basis for this heterogeneity is unknown. Here, we summarize some of the data illustrating this heterogeneity and speculate about possible mechanisms that may cause it. It could, for example, be intrinsic in the leukemia stem cell or could be related to some aspect of the patient's response to the leukemia.