The predominance and continual emergence of new variants in GII-4 noroviruses (NVs) in recent years have raised questions about the role of host immunity and histo-blood group antigens (HBGAs) in NV evolution. To address these questions, we performed a genetic and phenotypic characterization of GII-4 variants circulating in the past decade (1998 to 2008). Ninety-three GII-4 sequences were analyzed, and of them, 16 strains representing 6 genetic clusters were selected for further characterization. The HBGA binding properties were determined by both saliva-and oligosaccharide-binding assays using P particles as a model of NV capsid. The antigenic properties were also examined by enzyme immunoassay (EIA), Western blot analysis, and receptor blocking assay, using P-particle-specific antibodies from immunized mice and GII-4 virus-infected patients. Our results showed that 15 of the 16 GII-4 viruses bound to saliva of all A, B, and O secretors. Oligosaccharide binding assays yielded largely consistent results, although the binding affinities to some oligosaccharides varied among some strains. The only nonbinder had a mutation in the binding site. While antigenic variations were detected among the 16 strains, significant cross-blocking on the HBGA binding was also noted. Sequence alignment revealed high conservation of HBGA binding interfaces with some variations in adjacent regions. Taken together, our data suggested that the ability of GII-4 to recognize different secretor HBGAs persisted over the past decade, which may explain the predominance of GII-4 over other genotypes. Our data also indicated that both the host immunity and HBGAs play a role in NV evolution. While host immunity may continue driving NV for antigenic change, the functional selection by the HBGAs tends to lock the architecture of the capsid/HBGA interfaces and allows only limited variations outside the HBGA binding sites. A potential outcome of such counterselection between theses two factors in NV evolution is discussed.Noroviruses (NVs) have been recognized as the most important cause of nonbacterial acute gastroenteritis in both developed and developing countries, affecting people of all ages (13,35,39,44,48,56). They are single-stranded positive-sense RNA viruses belonging to the family Caliciviridae. NVs are highly contagious, spreading by a fecal/oral pathway through person-to-person contact and by contaminated food and/or water and usually causing large outbreaks within closed communities in a variety of settings, such as hospitals, nursing homes, schools, childcare centers, restaurants, cruise ships, and the military (11, 63). Human NVs have been difficult to study due to diverse members and the lack of an efficient cell culture and animal model for human NVs. The cloning of the NV genomes (33,36,73) and subsequent expression of the viral capsid proteins in baculovirus and other expression systems (3, 31, 32) have greatly advanced the research of NVs, including host-virus interaction, immunology, diagnosis, molecular virology, and epidemio...