The R5 HIV-1+ cells that we detected in the CCR5− TM subset might result from the presence of R5 HIV-1-permissive cells that were not detected for CCR5 expression by flow cytometry, as low levels of CCR5 are sufficient for R5 HIV-1 infection if they are associated with sufficient cell-surface expression of CD4 [32]. This evidence concerns the gene CD4 and HIV-1 infection.