Cover picture: Ribbon diagrams of virus hemagglutinin A (HA) trimers colored according to the size of motions in four different types of collective modes. The color code used in the order of increasing mobilities is: green (almost rigid), cyan, blue, magenta, pink and yellow (most mobile). Part a refers to the lowest frequency mode: a global torsion around a central hinge site in the stem region of HA. Part b refers to the second slowest mode, characterized by the large flexibility of the HA1 globular domains while the fibrous stem region is almost intact. Parts c and d illustrate the increase in the number of hinge sites (indicated by dashed lines) along the longitudinal axis, or the decrease in the size of the substructures that undergo coupled, coherent movements, as higher frequency modes are observed. See the article by Isin et al. on p. 569.