ABCB1 and neoplasm: The overexpression of the ABCB1 (MDR1) gene product P-glycoprotein (P-gp) has been identified as an important mediator of multidrug resistance (MDR) to chemotherapeutic agents, which can lead to increased tumour resistance and worse prognosis in cancer patients (Gottesman, 2002; Longley and Johnston, 2005).