基金
Supplementary information is linked to the online version of the paper on the Cell Research website. Acknowledgments We thank Yun Luo, Jia Jia and Dr Yun Xu at Nanjing University, Nanjing, China, for their scientific discussions and experimental assistance. This work was supported by grants from the National Natural Science Foundation of China （Nos. 30871195, 90813035, 30890044）, the National Basic Research Program of China （Nos. 2006CB503909, 2004CB518603）, the ＂111＂ Project, the Hi-Tech Research and Development Program of China （No. 2006AA02A112,）, and the Natural Science Foundation of Jiangsu Province （Nos. BK2004082, BK2006714, BK2008021）.
