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. Glycoprotein gp41 is embedded in the viral membrane and can interact with other gp41 proteins. The proteins gp41 and gp120 form a complex which is thought to undergo conformational changes in order to initiate membrane fusion of the virus with a cell.