A diverse panel of VLPs representing G1 and GII norovirus strains and epitope mutants was assembled as previously reported [13, 17, 75] . To design epitope exchange chimeric VLPs, we first identified surface exposed residue clusters that varied over time. Then, we synthesized a series of chimeric GII.4 ORF2 genes that exchanged ''putative'' epitopes between GII. 4.1987 and GII.4.2006 VLPs [43] . For all constructs except GII.4.2009 ORF2 [17] , the synthetically derived constructs were inserted directly into the VEE replicon vector for the production of virus replicon particles (VRPs) as previously described by our group. VLPs were expressed in VRPinfected BHK cells and purified by velocity sedimentation in sucrose and stored at 280uC. The GII.4.2009 (New Orleans [17] ) VLPs were expressed in the baculovirus system and purified by cesium chloride gradient centrifugation and were the kind gift of Dr. Jan Vinje, Centers for Disease Control and Prevention, Atlanta, GA. VLP protein concentrations were determined by the BCA Protein Assay (Pierce, Rockford, IL). VLP preparation purity averaged .80% by SDS-Page analysis.