Most of the research efforts in HIV vaccines have gone into the subunit vaccines involving the gp120 envelope proteins, which also includes the GP41 domain. Both of these proteins do elicit neutralizing antibodies to the homologous vaccine strain but not to heterologous primary isolates in the animal model. Despite these caveats, two large scale phase III clinical trials of this type of vaccine have been carried out involving 7,500 high-risk individuals. Neither of these trials showed a significant reduction in HIV infection in the vaccinated individuals. Neither of these trials showed a significant reduction in HIV infection in the vaccinated individuals, even with continuous booster shots during the three-year trial. A number of other trials involving trimeric gp140, a DNA vaccine encoding a V2-deleted gp 140 and other combinations, are in various phases, but the results are not known at present.