ABCB1 and neoplasm: Resistance against most chemotherapeutic agents is partly mediated by multidrug resistance protein 1 (MDR1) whose expression is higher in the invasive tumours (e.g., ccRCCs) compared with non-invasive kidney tumours, such as renal oncocytomas (ROs) and decreases in the more undifferentiated tumours, but still remains at levels high enough to be drug resistant [14, 15].