We thank Dr. Andrew S. Belmont for kindly providing the AO3_1 CHO cell line, and Dr. lan N Hampson (University of Manchester Gynaecological Oncology Labs, School of Cancer & Enabling Sciences) for kindly providing the C33A-E6 and C33A-C cells. This work was supported by the National Basic Research Program of China (2007CB914603), the National Natural Science Foundation of China (30970677), and the Outstanding Youth Scientist Foundation of National Natural Science Foundation of China (30825011).