Cross-sectional schematic diagram of HIV virion. Each virion expresses 72 glycoprotein projections composed of gp120 (orange) and gp41 (light blue). Gp41 is a transmembrane molecule that crosses the lipid bilayer of the envelope. Gp120 is noncovalently associated with gp41 and serves as the viral receptor for CD4 on host cells. The viral envelope also contains some host-cell membrane proteins such as class I and class II MHC molecules. Within the envelope is the viral core, or nucleocapsid, which includes a layer of a protein called p17 (green) and an inner layer protein called p24 (yellow). The HIV genome consists of two copies of ssRNA, which are associated with two molecules of reverse transcriptase p64 (light red) and nucleoid proteins p10, a protease (red), and p32, an integrase (dark blue).
