The use of CXCR4 and CD134 as receptors is compatible with our hypothesis, as well as, analogous to primate lentivirus receptor usage, the predominant FIVfca quasispecies changes during the course of FIV infection, in that isolates from terminally infected animals have been reported to be CD134 independent [168]. This evidence concerns the gene TNFRSF4 and infection.