Latently infected cells are thought to arise from infection of CD4+ T cells activated during an immune response, when the activated T cells, like their uninfected counterparts, return to an immunologically resting state (4), or from infection of activated CD4+ T cells in a narrow time window in the transition to resting memory cells (5), thereby establishing the reservoir in which HIV persists (6–10) as a barrier to cure (11–13). This evidence concerns the gene CD4 and infection.