Although Raji/CD4 cells are not natural targets for HIV-1 infection, we believe that they do provide an appropriate model system to study HTLV-1 cell-to-cell infection, as B-cells are natural targets for HTLV-1 infection in vivo and in vitro[35],[36],[37],[38],[39] and can form synapses with HTLV-1-infected T lymphocytes in vitro[14]. Here, CD4 is linked to HIV-1 infection.