represent spleen colony-forming stem cells (CFU-S day-12), cobblestone area whereas the cells giving rise to late appearing, more permanent CAs are related to primitive stem cells with long-term in vivo repopulating ability. 21,[24][25][26] Similarly, in the human CAFC assay both the phenotypes of the CAFC subsets 23 and the in vivo repopulation data in SCID mice 27,28 and men 29 are conIntroduction cordant with the assumption that CAFC wk 2-3 represent more transiently repopulating stem cells, whereas CAFC wk Chronic myeloid leukemia (CML) is a malignant disorder aris-5-8 are indicators of stem cells that induce stable chimerism ing from a pluripotent hematopoietic stem cell. 1 The disease in vivo. We have previously shown that the FBMD-1 stromal is characterized by excessive production of mature blood cells cell line allows long-term growth of AML leukemic stem belonging predominantly to the myeloid lineage. Malignant cells. 30 In the present study we have investigated the possicells are characterized cytogenetically by a translocation bility of propagating CML stem cells over an extended period between chromosomes 9 and 22 (t(9;22)) and, at the molecuof time in order to improve the analysis of their frequency and lar level, by the bcr/abl hybrid gene resulting from the transpoability to generate committed progenitors. sition of the c-abl proto-oncogene on chromosome 9 to the breakpoint cluster region (bcr) on chromosome 22. sent from CML patients in first chronic phase.