Proceeding inward, the viral matrix protein forms a layer just under the lipid bilayer. The structure of this layer has been controversial. Figure 1 shows one proposal, hexagonal packing of MA trimers (yellow)(Alfadhli et al. 2009). Movie 2 shows an MA hexamer bound to a modeled membrane. Lentivirus particles are distinguished by the presence of a bullet-shaped capsid (orange) assembled from monomers of the viral capsid protein (CA). The cone is a hollow "fullerene cone" built up from CA hexamers (Movie 3) assembled together with twelve CA pentamers (Ganser et al. 1999).