Given that FIV also targets primary CD4 T cells but uses CD134 instead of CD4 as a primary receptor and uses its sole co-receptor CXCR4 for efficient infection of target cells, similarly to T cell–tropic strains of HIV [18,19,49], FIV infection of cats is an ideal in vivo model to investigate CXCR4-mediated neuropathology in chronic HIV infection. This evidence concerns the gene CD4 and infection.