Agreeing with the assumption that epitope-exchange mutants are unlikely to identify epitopes of cross-reactive mAbs, NVB 37.10, 61.3 and 71.4 reacted with the entire panel of chimeric VLPs by EIA ( Figure 10B ). In contrast, each of the strain-specific mAbs displayed differential EIA reactivity to exchanged epitopes Table S4 ). Exchange of the other GII.4.1987 epitopes did not eliminate NVB 114 blockade potential ( Figure 12A and B) Figure 13A and B) . Binding was not restored to wild type levels as the EC50 of GII.4.1987/2006D was 0.6349 mg/ml, significantly higher than the blockade EC50 for GII.4.2006 -PGM (0.1195 mg/ml) ( Figure 13B and Table S4 ) (p,0.05). Both EIA and blockade data clearly indicate that Epitope D is critical for the binding of NVB 97 and suggest that amino acids 393-395 are important components of a GII.4 evolving blockade epitope in addition to modulating VLP-carbohydrate ligand binding [13, 21] . Interestingly, Epitope D has a single amino acid change in GII.4.2004, explaining the highly conserved binding and blockade responses noted across GII.4.2005 to 2009 VLPs, while ancestral strains display significant antigenic variation across these residues. Together, these data map the GII.4 evolving blockade epitopes recognized by each of the four strain-exclusive human mAbs described in this study ( Figure 14 ).