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HIV-1 and HIV-2
An article from "The Molecules of HIV" (c) Dan Stowell
By 1985 HIV was a recognised retrovirus, but then it was observed that the few AIDS cases in West Africa were associated with a different virus from what had become known as HIV. The new virus was called HIV-2.
Are they "the same" virus? Clearly they're related (they have similar effects and almost exactly the same set of genes) - but in fact HIV-2 is strictly more like certain types of simian immunodeficiency virus (SIV) than it is like HIV-1. Both HIV-2 and SIV carry an extra gene not found in HIV-1, which encodes Viral protein X. And comparing the viral proteins of HIV-2 and SIV, only subtle differences in molecular sizes can differentiate them.
All three - HIV-1, HIV-2, and SIV - seem to have developed from a common ancestor.
HIV-1 is more pathogenic than HIV-2. The reason seems simply that once it's integrated into a cell, it is faster at producing new virus particles.
See also: The HIV genome
Relevant journal articles: