CD4 and HIV-1 infection: In studies of HIV-1 infection, it has been demonstrated that the virus binds to DC-SIGN (CD209, a dendritic cell-specific C-type lectin expressed on the surface of dendritic cells) via its envelope protein gp120 and then spreads along the infectious synapses formed between CD209+ dendritic cells and CD4+ T cells, leading to efficient infection of CD4+ T cells by HIV-1.