ABCB1 and malignant colon neoplasm: 4) The combined treatment of 5AC with TSA increased MDR1 mRNA expression either additively 20% (SNU-5 and -638) or synergistically 40% (SNU-16, -601, -668 and -719) in the gastric cancer cells but only synergistically in the SNU-C5 colon cancer cells.