The capsid of spherical viruses is built from a limited number of proteins and often displays icosahedral symmetry. Rotaviruses have a segmented double-stranded RNA genome enclosed in a complex capsid formed by three concentric protein layers. The proteins forming the capsid are VP2 (internal layer, with triangulation T = 1 and an asymmetric dimer in the icosahedral repeating unit), VP6 (intermediate layer, T = 13 symmetry), VP7 (external layer, T = 13) and VP4, which forms a spike inserted in the outermost two layers. The major capsid protein VP6 self-assembles into spherical or helical particles mainly depending upon pH. VP6 assemblies arise from different pickings of a unique dimer of trimers. The repeating unit of the helix contains a pair of trimers related by a radial dyad . The VP6 trimer is composed of two domains: a head (external) and a base (internal), leaving a central cavity, these are formed by a distal beta-barrel domain and a proximal alpha-helical domain, which interact with the outer and inner layer of the virion, respectively .