Advantage of AKR/J thymocytes is not due to increased proliferation capability. Mixed BM chimeras reconstituted with 50% of AKR/J inoculum were analyzed for in vivo incorporation of BrdU at 3 and 6 weeks after transfer. BrdU (1 mg) was twice injected intraperitoneally at 4-hour intervals, and analysis was performed 1 hour after the last injection. (A) Thymocytes were analyzed by 4-color staining by using biotin–anti-Thy1.1, PE–anti-Thy1.2, FITC–anti-BrdU, and 7-AAD. FACS profiles show BrdU versus 7-AAD staining in total thymocytes and in gated Thy1.1⁺ or Thy1.2⁺ cells. (B) Thymocytes were analyzed by 4-color staining by using a cocktail of PE–(anti-CD4, anti-CD8, anti-TCRαβ) antibodies together with FITC–anti-BrdU, 7-AAD, and biotin–anti-Thy1.1 or biotin–anti-Thy1.2. FACS profiles show BrdU versus 7-AAD staining in gated TN (CD4^–CD8^–TCRαβ^–) thymocytes and in gated Thy1.1⁺ or Thy1.2⁺ TN thymocytes. Results (A-B) are representative of 4 independent experiments with n = 3 mice per group of BM chimeric mice. Numbers shown in FACS profiles denote percentages of cells that fall into each quadrant.