Among the various molecular mechanisms associated with MDR in experimental tumour models, one of the most extensively studied involves decreased drug accumulation due to enhanced efflux by ATP-dependent transporter proteins such as P-glycoprotein (Pgp), the human multidrug resistance-associated protein 1 (MRP1), and the recently described breast cancer resistance protein (BCRP). This evidence concerns the gene ABCG2 and neoplasm.