Although Env engagement of CD4 at the cell surface is required for VS formation, the expression of Env at the cell surface renders infected cells susceptible to antibody detection and, while many antibodies against Env can block the formation of virological synapses, they are less efficient at blocking cell-to-cell infection than they are at blocking cell-free infection [8,9,10,11,12]. This evidence concerns the gene ERVW-1 and infection.