Two of the most widely studied efflux pumps, MDR-1 P-gp/P-170, the gene product of MDR-1 (ABCB1) and MRP-1 (ABCC1) which encodes a 190 kDa membrane protein have both been demonstrated to pump a wide variety of the most commonly used cancer drugs out of tumour cells. This evidence concerns the gene PGP and neoplasm.