Baboons have also been evaluated for their ability to support infection with simian-human immunodeficiency viruses (SHIVs) made with a SIVmac239 backbone (Allan et al., 1995), however these SHIVs expressed Env from CXCR4-or dual-tropic HIV-1 strains, distinct from the CCR5-tropic nature of SIVs. This evidence concerns the gene CCR5 and infection.