Because functional ABCG2 has been reported to be overexpressed on the surface of primary CML stem cells [29], we combined surface marker characteristics with Hoechst dye efflux to explore the interaction of the PI3K/Akt pathway, ABCG2 expression and the SP fraction specifically in the primitive stem cell subset. This evidence concerns the gene ABCG2 and chronic myelogenous leukemia, BCR-ABL1 positive.