A staging system was proposed for chronic myeloid leukemia (CML) that groups patients into three stages (I, II, and III), each with two subclasses (A and B). The system uses pathologic parameters, of which the percentage of blasts in the blood and bone marrow is the most important. CML is defined as a myeloproliferative disorder with molecular or cytogenetic evidence of the translocation of the abl oncogene on chromosome 9 to the break-point cluster region gene on chromosome 22. The proposed staging system is similar to format to the standard TNM system used for many solid tumors.