The vaccine Env 2-3 is created from one of the viral proteins that make up HIV called envelope glycoprotein gp120. A problem with many immunogens, including candidate HIV vaccines, is that they may evoke relatively weak immune responses, particularly in humans and in nonhuman primates. Thus, there is considerable interest in the development of "adjuvants" (substances that augment immune responses to vaccines). MTP-PE/MF59 is an adjuvant that appears to be particularly promising, and is selected for the studies with this HIV vaccine candidate.