Competitive bone marrow chimeras indicate CCR4 is required for negative selection. (A) Experimental schematic for the generation of competitive bone marrow chimeras. Thymic and lymph node chimerism was analyzed by flow cytometry 6 wk after transplantation. (B) Representative CD45.1 versus CD45.2 and CD4 versus CD8 flow cytometric plots from the indicated recipient thymi. (C) The ratios of CD45.2/CD45.1 cellularity for each specified thymocyte and lymph node subset were calculated and normalized to the DP CD69⁻ population. Data are compiled from four experiments with a total of 11 mice per group. **, P < 0.01; ***, P < 0.001; ****, P < 0.0001 (unpaired Student’s t test).