Ruan Yuhua,Division of Virology and Immunology, National Center for AIDS/STD Control and Prevention, Chinese Center for Disease Control and Prevention, 27 Nanwei Road, Xuanwu District, Beijing 100050,China.Email: yh_ruan@sohu.com; yshao@public3.bta.net.cn SHAO Yiming, Division of Virology and Immunology, National Center for AIDS/STD Control and Prevention, Chinese Center for Disease Control and Prevention, 27 Nanwei Road, Xuanwu District, Beijing 100050,China.Email: yh_ruan@sohu.com; yshao@public3.bta.net.cn