Most of the investigated chalcones showed the ability to inhibit three well-characterized members of the ATP-binding cassette (ABC) transporter family: P-glycoprotein (P-gp, ABCB1), multidrug resistance-associated protein 1 (MRP1, also known as ABCC1), and breast cancer-resistance protein (BCRP, ABCG2), main contributors to the MDR in cancer cells. This evidence concerns the gene ABCG2 and cancer.