ABC (ATP-binding cassette) transporters constitute a superfamily of highly conserved transmembrane proteins that convert ATP into energy and use that energy to import and export substances in and out of cells. As such, they are important for most aspects of cell physiology including uptake of nutrients and export of waste products. The members that are extensively studied include the multidrug resistance-1 (MDR1) gene product, P-glycoprotein (P-gp, ABCB1), and the breast cancer resistance protein (BCRP, ABCG2) because of their natural ability to export therapeutic drugs, thereby restricting the bioavailability of administered drugs. The recent molecular characterization of blood groups Junior (Jr) and Langereis (Lan) has revealed that they belong to ABC transporter superfamily of proteins. In this session, the studies that have led to the identification of two blood group antigens will be described and the potential clinical implications for individuals with the null alleles will be discussed. Various ABC transporters are highly expressed in hematopoietic stem cells (HSCs), including both normal and malignant HSCs. Insight from ex vivo manipulation and experimental models as to the role of these transporters in cancer stem cells will be discussed.