The rotavirus virion first attaches to the target cell; many strains bind cell surface sialic acids through VP8* (produced by cleavage of VP4 into VP5* and VP8*) at the tips of the virion spikes. Non-clathrin-, non-caveolin-mediated endocytosis delivers the virion to the early endosome. There, reduced calcium concentrations are thought to trigger uncoating (loss of VP7) of the triple-layered particle (TLP) and membrane penetration by VP5*. Loss of the outer capsid and release of the double-layered particle (DLP) into the cytosol activates the internal polymerase complex (VP1 and VP3) to transcribe capped positive-sense RNA ((+)RNAs) from each of the 11 double-stranded RNA (dsRNA) genome segments. (+)RNAs serve either as mRNAs for synthesis of viral proteins by cellular ribosomes or as templates for synthesis of negative-sense RNA ((–)RNA) during genome replication. Non-structural protein 2 (NSP2) and NSP5 interact to form large inclusions (viroplasms) that sequester components required for genome replication and the assembly of subviral particles. Genome packaging is initiated when VP1 (and, presumably, VP3) bind the 3′ end of viral (+)RNAs. It is currently thought that interactions among the 11 (+)RNAs drive formation of the 'assortment complex'. Condensation of the inner capsid protein, VP2, around the assortment complex triggers dsRNA synthesis by VP1. The intermediate capsid protein, VP6, then assembles onto the nascent core to form the DLP. Assembly of the outer capsid is not well understood; the current model proposes that interaction with the rotavirus transmembrane protein, NSP4, recruits DLPs and the outer capsid protein VP4 to the cytosolic face of the endoplasmic reticulum (ER) membrane. Through an undefined mechanism, the DLP–VP4–NSP4 complex buds into the ER. Subsequent removal of the ER membrane and NSP4 permits assembly of the ER-resident outer capsid protein, VP7, and formation of the TLP (see Fig. 6). Release from the infected cell exposes the virion to trypsin-like proteases of the gastrointestinal tract, resulting in the specific cleavage of VP4 into VP5* and VP8* to produce the fully infectious virion. IFN, interferon. IFN, interferon. Modified, with permission, from the following website