Many retroviruses carry no genetic material other than these three genes. Some of the products of these genes also may alter the activity of the infected cells. Since the gag , pol , and env genes of HIV are rather typical of all retroviruses, it is these regulatory genes that give the virus much if not all of its lethality. Finally, the ends of the HIV genome include two regions called long terminal repeats, or LTRs. These regions do not produce protein, but they do control the expression and function of the viral genes when some of the products of HIV's regulating genes attach to points within the LTR themselves.
Viruses vary enormously in relative size.
The largest, the pox and herpes viruses, can carry 100 genes or more and have a highly complex structure. In contrast, the tiniest, such as those that cause polio and hepatitis A, are so small that there is space inside their simple protein shell for no more genetic material than that in a typical cellular gene. Clearly, size and genetic endowment has nothing to do with a viruses' ability to cause disease. The numbers reflect the relative size of the proteins. Enveloped viruses, which includes HIV and all other retroviruses, have an outer surface composed of membranes from the cells they infect.