Of the two types of Human immunodeficiency virus (HIV) which have been characterized, HIV-1 is more infective and is prevalent globally. The HIV virion is encapsulated by a viral envelope that contains complexes of surface proteins, which are important for the virus to attach and fuse to infect target cells. A precursor molecule known as gp160 (glycoprotein 160) is cleaved by proteases into gp120 (glycoprotein 120) and gp41 (glycoprotein 41), glycoproteins that make up the envelope proteins of HIV. The gp120 protein is composed of five variable regions (V1-V5) which often form loop structures, and five constant regions (C1-C5). The third variable loop (V3 loop) of gp120 interacts with cytokine receptors to mediate the entry of the virus into cells. This antibody, clone 2558, is directed against the V3 loop of gp120 and has been shown to neutralize HIV-1 primary isolates.