CD4 and infection: Because a self-propagating and expanding infection requires virus production, we used an unambiguous assay to document HIV-producing CD4+CD25– T cells as the nearly exclusive source of virus in Fiebig I. We also focused on pTEFb as essential for virus production, with the hypothesis that pTEFb would be expressed in CD4+CD25– T cells in vivo in LT in contrast to resting T cells in vitro (33), and found that HIV replicates in resting T cells in vivo that do express pTEFb in LT.