HIV virus. Cutaway illustration of a human immuno- deficiency virus (HIV), the cause of AIDS (acquired immune deficiency syndrome). In the core are two strands of RNA (ribonucleic acid, orange), which carry the viral genes. Associated with these is the enzyme reverse transcriptase (red), which inserts the genes into the host cell's DNA (deoxyribonucleic acid). The RNA is enclosed in two protein coats (blue and mauve) made of repeating subunits called capsomeres (spheres). Around these is the membrane, a lipid bilayer (orange), which is studded with GP120 glycoproteins (green) that bind to target human white blood cells.