The ATP-binding cassette (ABC) proteins ABCB1 [P-glycoprotein (Pgp); MDR1] [18] and ABCG2 [breast cancer resistance protein (BCRP); mitoxantrone resistance protein (MXR)] [19] are drug efflux proteins that are frequently expressed on AML cells. Here, ABCG2 is linked to acute myeloid leukemia.