PGP and cancer: Multidrug resistance (MDR) is defined as the development of resistance by cancer cells to different chemotherapeutic agents, and the ATP-binding cassette (ABC) transporter superfamily, such as P-glycoprotein (P-gP: ABCB1\/MDR1), ABCG2 (breast cancer resistance protein: (BCRP); ABCG2), and multidrug resistance-associated protein 1 (MRP1, (ABCC1)), are involved in this resistance.