Viruses use host cells to replicate and progeny virions that can infect other cells. These released virus particles contain the viral genome as well as a capsid, the protein shell that protects and houses the viral genome. In the case of HIV, a single viral capsid is composed of more than 3000 copies of the HIV Gag protein and encapsidates two copies of HIV genomic RNA. Gag proteins are translated in the cytoplasm (Fig. 1A), and subsequently target to the plasma membrane (Fig. 1B), where they assemble into the spherical immature capsid (Fig. 1C). This assembly process is complex, since it involves selective packaging of the viral genome and specific viral proteins. The immature capsid is subsequently released from the cell by budding from the plasma membrane (Fig. 1D), a process that is genetically and biochemically distinct from the events of immature capsid assembly. In the last step of virion formation, the Gag proteins in the immature capsid are cleaved by the HIV protease allowing formation of the mature virus particle (Fig. 1E).