ABCG2 is overexpressed in CML cells. ABCG2 is normally highly expressed in CD34⁺ CD38^– human hematopoietic stem cells and switched off in most maturing lineages including granulocyte/monocytes. The data from Jordanides et al's paper suggest that in CML, the expression of this transporter persists further down the maturation spectrum, such that it may be found on the majority of CML CD34⁺ cells, including those beyond the CD34⁺ CD38^– stage, and maybe even those that coexpress promyeloid markers including CD13 and CD33. Thus their findings are consistent with the possibility that a greater proportion of cells have the active transporter rather than the per-cell level of ABCG2 being elevated. SC indicates multipotent stem cell/hemangioblast; HSC, hematopoietic stem cell; HPC, hematopoietic progenitor; CPC, CML progenitor; and ProM, promyeloid. Blue shading indicates ABCG2 expression levels. Diagram kindly provided by Dr J. Mountford, Royal Infirmary, Glasgow, United Kingdom.