In previous study, we found that the low concentration of ophiobolin O (0.1 μM) down-regulated the expression of resistance-related protein P-Glycoprotein (P-gp, also known as MDR1), which makes Adriamycin-resistant human breast carcinoma (MCF-7/ADR) cells more sensitive to Adriamycin treatment. The gene discussed is ABCB1; the disease is breast carcinoma.