In addition, similar to the enhancement of X4 virus infection by prestimulation of CXCR4 (Figure 1C) (Yoder et al., 2008), CXCR4 prestimulation also enhanced the latent infection of memory CD4 T cells by the R5 virus (Figure 1F), which is in great contrast to the potent inhibition of HIV latent infection of memory T cells by CD2 prestimulation. Here, CD4 is linked to disease arising from reactivation of latent virus.