Expression of SIV and HIV-1 coreceptor CCR5 (bottom panels) and HIV-1 coreceptor CXCR4 (top panels) on the thymocytes. Data for the CD4+ single-positive cells, the CD4/CD8 double-positive cells, the CD8+ single-positive cells, and the CD4/CD8 double-negative cells are indicated with the appropriate labels above the panels. The double-negative subpopulation might include cells other than thymocytes. Percentages of expression are also indicated within the panels. The thymocytes were labeled with FITC-conjugated anti-CD8 antibody and peridinin chlorophyll protein-conjugated anti-CD4 antibody to obtain the gated subpopulations and were labeled with PE-conjugated antibodies to CXCR4 or CCR5.