Many neutralizing epitopes have been mapped in the variable regions of the proteins of viruses, including infectious bursal disease virus [25] , infectious bronchitis virus [26] , hepatitis C virus [27] , and HIV [28] . Some neutralizing epitopes, however, are highly conserved across most of the viruses in the same group [29, 30] . A novel epitope was mapped within the highly conserved flavivirus fusion loop peptide 98 DRXW 101 by phage-display biopanning and structure modeling using mAb 2A10G6 that had broad cross-reactivity with dengue virus (DENV) 1-4, yellow fever virus (YFV), West Nile virus (WNV), and Japanese encephalitis virus (JEV) viruses. This mAb potently neutralized DENV 1-4, YFV, and WNV and conferred protection against lethal challenge with DENV 1-4 and WNV in murine model. Further functional studies revealed that 2A10G6 blocked infection at a step after viral attachment. These results show that the broad cross-reactivity epitope recognized by neutralizing mAb 2A10G6 is highly conserved among DENV 1-4, YFV and WNV [29] . An epitope recognized by mAb 51 belonging to isotype IgM was mapped to 215 KQEKD 219 of the VP1 capsid protein of Enterovirus 71 (EV71), which possessed neutralizing activity in vitro and provided 100% in vivo passive protection against lethal challenge with EV71 strain HFM 41. BLAST analyses of the neutralizing epitope revealed that it was highly conserved among all EV71 strains, but not coxsachievirus 16 [30] . In this study, the epitope recognized by neutralizing mAb A9E8 was mapped to a highly conserved region of the gp90 protein among REVs, which would be useful for development of REV marker vaccines and diagnostic techniques.