 HIV targets the immune system by attacking certain T cells called CD4 T cells. There are parts on the AIDS viruses that help the virus infect these cells and other parts that help the immune system prevent viral infection by activating protective T-cells that fight HIV. Different T-cell populations are very important in most vaccines as they act as effectors that work as part of the immune system to recognize and fight off HIV infection. When effector T cells are activated by appropriate protective parts of the virus they either block HIV from reproducing or kill HIV infected cells. By finding these common protective parts of each of these human and animal AIDS viruses, the investigator hopes to make a vaccine that helps the immune system prevent HIV infection by avoiding parts that attack CD4 T cells and may worsen HIV infection and selecting for parts that stimulate effector T cells that fight HIV infection. 