In comparing gene expression of normal and CML CD34+ quiescent (G0) and proliferating (G1/S/G2/M) cells, 292 genes were down-regulated and 192 genes were up-regulated in the CML G0 cells. The differentially expressed genes were grouped according to their reported functions and correlations were sought with biological differences previously observed between the same groups. The most apparent correlations include: i) Normal and CML G0 cells are more primitive than G1/S/G2/M cells; ii) CML G0 cells are in a more advanced stage of development and more poised to begin proliferating than normal G0 cells; iii) When CML G0 cells are stimulated to proliferate, they undergo further differentiation and maturation more rapidly than normal G0 cells, but both granulopoiesis and erythropoiesis are less efficient than normal; iv) Whereas normal G0 cells form only granulocyte/monocyte (GM) colonies when stimulated by cytokines, CML G0 cells consistently form a combination of GM and erythroid clusters and colonies; and v) Prominin-1 (CD133) is the gene most down-regulated in CML G0 cells and its down-regulation appears to be associated with the spontaneous formation of erythroid colonies by CML progenitors without EPO. The gene most over-expressed in CML G0 cells is LepR, but its role in contributing to the myeloid expansion and other abnormalities is unknown. It was hoped that LepR might serve as a therapeutic target, but leptin had no stimulatory or inhibitory effect on either normal or CML G0 cells, our attempts to make a specific LepR antibody were unsuccessful, and no other potentially targetable over-expressed surface antigens were identified.