ABCG2 and cancer: Multi-drug resistance protein 1 (MRP-1; (Cole et al, 1992)) is one of the first described ABC transporter proteins, expressed in the plasma membrane of normal and cancer cells where it effluxes both physiological toxins (Leier et al, 1994; Jedlitschky et al, 1997; Haimeur et al, 2004) and a variety of agents from the cell (Grant et al, 1994; Loe et al, 1998; Renes et al, 1999), leading to decreased cytotoxicity.