HIV-1 infection of CER and VAG cells was determined by dual intracellular staining for HIV-1 Gag and epithelial marker cytokeratin 19 (CK19) proteins after exposure to HIV-1 (IIIB or Bal)–HTLV-1-coinfected primary CD4+ T cells (Fig. 2B and C, two left panels) or T cell lines (Fig. S3A to C, left panels). This evidence concerns the gene CD4 and HIV-1 infection.