Chronic myeloid leukemia (CML) is a hematopoietic stem cell disorder that is 
characterized by splenomegaly and marked elevation of the blood leukocyte count 
with granulocyte in maturity. Ph chromosome was identified in CML in 1960 and 
was found to clearly result from reciprocal translocation between chromosome 9 
and chromosome 22 (t(q;22)) (q34;q11). CML arises from a single pluripotent 
hematopoietic stem cell with the Ph chromosome and demonstration of the Ph 
chromosome in blood or marrow cells establishes and unequivocal diagnosis of 
CML. The Ph chromosome is recognized as the cytogenetic result of a 
rearrangement of the ABL gene on chromosome 9 and the BCL gene on chromosome 22, 
which leads to the creation of a BCR/ABL fusion gene on chromosome 22. Abnormal 
ABL-related protein with increased tyrosine kinase activity suggested a 
molecular mechanism of CML. The BCR/ABL fusion gene can be found not only in the 
chromosome but in interphase nuclei by fluorescence in situ hybridization 
(FISH). We employed both fluorescence activated cell sorter (FACS) and FISH to 
study the lineage involvement of individual stem cells and progenitor cells in 
patients with CML. Evidence of BCR/ABL fusion was found in pluripotent stem 
cells (CD34+, Thy1+), myeloid cells, B progenitor cells (CD34+, CD19+) and T/NK 
progenitor cells (CD34+, CD7+, CD5+) but not mature T cells (CD3+) or natural 
killer cells (CD3-, CD56+). These data suggested that BCR/ABL gene fusion occurs 
in pluripotent stem cells and that Ph+ T cells and natural killer cells are 
eliminated during differentiation.