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. The gene discussed is ABCB1; the disease is acute myeloid leukemia.