(a) Noroviruses are icosahedral shaped particles with a diameter of ca. 40 nm. The particles consist of 180 copies of the capsid protein VP1 forming 90 dimers (left: from yellow N-terminus to red C-terminus). Each VP1 consists of an N-terminal S-domain (yellow) and a C-terminal P-domain (red). The sketches are based on crystal structure data (PDB 1ihm). (b) Human norovirus P-dimers bind to HBGAs at the outermost tips at the interface. Shown here is the complex of a GII.4 Saga P-dimer with a blood group B trisaccharide (PDB 4 × 06, see also Figure 5 for the chemical formula).