HIV-1 cell-cell transmission was ~11-fold more efficient than free HIV-1 infection when transmission was measured from 293T cells to lymphoid cell line SupT1.CCR5, but cell-cell transmission was approximately threefold less efficient than free HIV-1 infection when we used the adherent target cells Cf2Th-CD4/CCR5 (Fig. 2E). Here, CCR5 is linked to HIV-1 infection.