1. College of Life ScienceNorthwest University Xi'an Shaanxi province 710069 P. R. China
2. CAS Key Laboratory of Pathogenic Microbiology and ImmunologyInstitute of MicrobiologyChinese Academy of Sciences 100101 Beijing P. R. China
3. Department of Computer Science and TechnologyXidian University Xi'an Shaanxi province 710069 P. R. China
4. Department of PathologyJohns Hopkins University Baltimore MD 21287 USA