1. Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development; Bethesda MD 20892-4417 USA
2. Health Science Center, Peking University; Beijing 100191 China
3. School of Biomedical Sciences, Faculty of Medicine, The Chinese University of Hong Kong; Shatin Hong Kong
4. National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health; Bethesda MD 20894 USA