Two types of HIV have been characterized: HIV-1 and HIV-2. HIV-1 is the virus that was initially discovered and termed both LAV (Lymphadenopathy Associated Virus) and HTLV-III (Human T cell Lymphotropic Virus III). HIV-1 is more virulent and more infective than HIV-2,  and is the cause of the majority of HIV infections globally. The lower infectivity of HIV-2 compared to HIV-1 implies that fewer of those exposed to HIV-2 will be infected per exposure. Due to its relatively poor capacity for transmission, HIV-2 is largely confined to West Africa. 
Nef also interacts with SH3 domains. The Vpu protein (p16) influences the release of new virus particles from infected cells.  The ends of each strand of HIV RNA contain an RNA sequence called the long terminal repeat (LTR). Regions in the LTR act as switches to control production of new viruses and can be triggered by proteins from either HIV or the host cell. The Psi element is involved in viral genome packaging and recognized by Gag and Rev proteins. The SLIP element (TTTTTT) is involved in the frameshift in the Gag-Pol reading frame required to make functional Pol. 
The term viral tropism refers to the cell types a virus infects. HIV can infect a variety of immune cells such as CD4 + T cells , macrophages , and microglial cells. HIV-1 entry to macrophages and CD4 + T cells is mediated through interaction of the virion envelope glycoproteins (gp120) with the CD4 molecule on the target cells' membrane and also with chemokine co-receptors.