Acknowledgments The authors are indebted to Dr. Li-Xin Li (Oregon University, USA), Dr. He-Sheng Ou (Baylor College of Medicine, USA) and Dr. Zhen-Gao Liu (Ohio State University, USA) for their critical discussions, suggestions, and valuable help in the preparation of the manuscript. This work was supported by the National Natural Science Foundation of China (30470719, 30600249), the National Major Basic Research Program of China (973) (2006CB503808), and the Education Department of Hunan Province (99B11).