The TZM-bl cell line is a derivative of HeLa cells that express CD4, CCR5, and CXCR4 and contains an integrated reporter gene for firefly luciferase and E. coli β-galactosidase under the control of an HIV-1 long terminal repeat (LTR) [29], permitting sensitive and accurate measurements of infection. This evidence concerns the gene CXCR4 and infection.