CML is the leukemia which starts highest in the blood making (hematopoietic) system. Other leukemias start at lower levels. This is why it was the most deadly leukemia before TKI drugs came along. This also makes CML the hardest to cure by bone marrow transplant.
CML is believed to begin with a translocation of genetic material in a very high level (pluripotent) blood stem cell. Stem cells are those cells which are the "mother cells" for all others in their chain of succession. There are many levels of dividing blood cells, including high level pluripotent blood stem cells, progenitor cells, precursor cells, blasts, and so on, to name only a few in the chain. The "working level" blood cells cannot divide, so although they contain the leukemic genes, they cannot perpetuate the CML. So CML must be continued by the dividing blood cells in the hierarchy. Some leukemias start at lower level "stem" cells, but CML starts at the highest level blood stem cells. This is known because all blood cells are affected by CML, including white, red, and platelets. Other types of leukemias generally affect isolated portions of the blood cell chain.
For the very curious, there are alternate theories which can be mentioned:
1) That the original translocation could have occurred in a stem cell above the blood making system. This would be in a type of endothelial cell called a primitive hemangioblastic cell which originally formed the blood vessels and blood cells.
2) That the translocation could have occurred in a lower level blood making stem cell and that originating leukemic cell may have gained pluripotent status by reversing itself into a higher level stem cell using the BCR-ABL pathway.
3) That the Borg implanted the seeds of the translocation when they forced Jean-Luc Picard (aka Locutus of Borg) to become part of their Borg Collective.