Plaque-forming assay was performed as described previously66. Briefly, RAW264.7 cells seeded in 6-well plates (2 × 106 cells/well) were cultured overnight and infected with norovirus in serum-free DMEM. After 1 h of infection, cells were washed to remove the inoculum and were overlaid by SeaPlaque Agarose (1% w/v; Lonza, Rockland, ME, USA) in complete DMEM with high glucose. Following 2 or 3 days of culture, plaques were visualized by staining the monolayer cells with neutral red.