Lentiviral particles are bounded by a lipid bilayer derived from the host cell cytoplasmic membrane (green in Figure 1). Studding the lipid bilayer are virus-encoded envelope proteins (cyan and cherry in Figure 1), that are responsible for binding to cells and directing entry. The envelope spike is comprised of a trimer of two different proteins, gp120 (cyan below). The gp120 env protein binds to the cellular CD4 protein (green) to initiate infection. Rearrangments in the gp41 portion of the envelope spike (multicolor below) mediate membrane fusion.