CD338, known as ABCG2 (BCRP1), is a multi-drug resistance (MDR) family protein that is a member of ATP binding cassette (ABC) transporters. It is highly expressed on primitive ‘side population’ (SP) stem cells. This SP phenotype is based on the efflux of fluorescent dyes such as Rhodamine 123 and Hoechst 33342. The SP cells express low or undetectable levels of CD34. In the bone marrow, about 0.05% population of cells displays the low fluorescence and are highly enriched for repopulating cells. The highest levels of BCRP1 mRNA expression have been seen in KDR+ human stem cells. The expression of ABCG2 appears to be highly conserved as it has been identified in various species.