1. Beijing Key Laboratory for HIV/AIDS Research, Clinical and Research Center for Infectious Diseases, Beijing Youan Hospital, Capital Medical University, Beijing 100069, China
2. Center for Infectious Disease Research, School of Medicine, Tsinghua University, Beijing 100084, China
3. Tsinghua-Peking Center for Life Sciences, Beijing 100084, China
4. Department of Microbiology & Molecular Genetics, Michigan State University, East Lansing, MI 48824, USA.