Hematopoietic stem cells are known to express a membrane transporter molecule, known as P-glycoprotein (Pgp), that is encoded by the multidrug resistance gene 1 (MDR1) (1, 2). Expression of Pgp appears to confer a proliferative advantage to stem cells through its anti-apoptotic effects (3, 4). A recent finding provides evidence that an additional transporter molecule known as Bcrp1 (Breast cancer resistance protein 1) or ABCG2 (ATP-binding cassette gene 2), first identified in a breast cancer cell line (5), is expressed on stem cells (6). Bcrp1/ABCG2 belongs to a family of molecules that span the cell membrane six times and can exist as either homo or hetero dimers linked by a short intracellular flexible linker region that plays an important role in the efflux of a wide range of substrates (7). Although these transporter molecules have initially been thought to play a role in drug resistance, lately they have been found to have utility in better characterizing primitive stem cells. For example, the “side-population” of hematopoietic stem cells, characterized by their inability to retain high levels of the intracellular staining dyes Hoechst 33342 and Rhodamine 123, has been found to express high levels of Bcrp1/ABCG2. Of interest is the observation that Bcrp1/ABCG2 function has been linked to the efflux of the Hoechst dye (6). Furthermore, there is now evidence that this monoclonal can be used as a cell surface marker to identify hematopoietic stem cells within the bone marrow fraction of lineage negative cells (6). The expression of Bcrp1/ABCG2 appears greatest on CD34--cells and is downregulated with the acquisition of CD34 on the cell surface (6). The ATP binding cassette transporter gene literature and how it impacts stem cell research has been recently reviewed by K. Bunting (8).