] TRIM5-alpha stymies HIV but not SIV, which explains why
] monkeys can contract only one of the two viruses. "The
] maintenance of a strong block to HIV-1 in Old World
] monkeys implies a selective advantage, presumably imposed
] by the presence of HIV-1-like viruses during the
] evolution of this primate lineage," the scientists write,
] adding that although humans carry a version of
] TRIM5-alpha, it is not as effective in thwarting HIV
] transmission.

Science & Technology at Scientific American.com: Monkey Protein Blocks HIV