This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
How does MDR gene act? 
Select and explain. 
A. Block drug activation, B. Block intracellular DNA synthesis, C. Cause efflux of drug, D. DNA repair inhibition
C: Cause efflux of drug
Answer: C: Cause efflux of drug 
Explanation: The multidrug resistance (MDR) gene encodes a protein that actively pumps drugs out of tumor cells. This gene confers resistance on a variety of antitumor drugs, including the antibiotics and plant-derived compounds. Tumor resistance to a given chemotherapeutic agent can often be overcome by the administration of multiple drugs. Ref: Sabel M.S. (2010). Chapter 44. Oncology. In G.M. Dohey (Ed), CURRENT Diagnosis & Treatment: Surgery, 13e.