ABCB1 and neoplasm: Overall, as shown in Table 3, a direct comparison of MDR1/P-gp and MRP-1 efflux pump expression in the primary vs metastatic specimens indicated a significantly higher percentage of metastatic tumours, when compared with primary tumours, expressed MDR1/P-gp (primary 50% metastases 74% P=0.010) and MRP-1 (primary 57% metastases 91% P⩽0.0001), with MRP-1 being the most prevalent efflux pump in these specimens.