The reporter cell line TZM-bl, expresses CD4, CXCR4 and CCR5 receptors; thus, its expression of these receptors is similar to that on the HIV-1 target cell, CD4+ T cell, and is suitable for evaluating infection by R5, R4, or dual R5 and R4 HIV-1 viruses (SF162, R5; DH12, dual R5 and R4). This evidence concerns the gene CD4 and infection.