X4 HIV-1 replicates in immature thymocytes, while R5 HIV-1 replicates in mature thymocytes in HIV-1-infected SCID-hu mice. Thymocytes were obtained from HIV-1-infected implants in SCID-hu mice at 19 days (NL4-3) and 26 days (JR-CSF) postinfection. The cells were surface stained with antibodies to CD3, CD71, CD69, CD27, and CD45RA combined with intracellular staining for GagHIV-1 proteins (KC57-FITC). The phenotype of GagHIV-1-expressing cells was determined by gating on KC57+ cells. The percentages of positive cells in each quadrant are indicated. Isotype controls were used to set the cursors. Results for one representative staining of two thymic implants at these time points are shown.